Most Rangers are equipped with the Ford 7.5-inch rear axle with several exceptions: 

1) 1986-up "Incomplete vehicles" also known as "Chassis Cabs" were frequently equipped with 8.8" 28-spline axles, the '86-88 examples are distinctive as they are the only 8.8" Ranger rears that have no provisions for a RABS sensor.

2) 1990 & up 4.0-liter Rangers. These are ALWAYS equipped with the 8.8-inch 28-spline rear axle.

3) 1998-up 4x4 supercab Rangers regardless of powerplant are frequently equipped with 8.8" axles

4) The FX4 Ranger was introduced in 2002 and always came with 4.10 gears only. The 31-spline Torsen 8.8-inch axle came only with the 2002 FX4 and 2003-up FX4 Level II and more recently was a separate option. The 2003-up FX4 (non-Level II) is essentially the same as the 1998-2002 Off-Road except that a regular Ford limited-slip in a regular 8.8-inch was included.
 
Swapping in an 8.8-inch 28-spline Ranger axle for the original 7.5-inch axle provides (according to Ford) a 35% increase in strength.  But this strength increase refers to the greater strength of the gears and carrier bearings (Pinion bearings, axle bearings and axle shafts are exactly the same) there is NO increase in weight capacity If you are looking for greater strength for loads created by severe offroad driving or towing you'll want to go to the next step, the Explorer axle.
 
The rear axles on all Ford Explorers are also 8.8's, however there are some differences...  The 8.8-inch Explorer axle use 31-spline axle shaft.  Increasing from a 28-spline to a 31-spline axle allegedly increases strength of the axle shaft by 34%, however I'll go by the capacity ratings of the axle assemblies provided by the manufacturer.  Ford Rates the Ranger axle at 2750lbs and the Explorer axle at 3200lbs this is a 16% increase in strength, which admittedly may be as conservative  
as the estimate of axle strength.  The Important thing about the Explorer axle assembly vis-a-vie it's strength is not the axles that are larger at the splines but that these axles are larger in diameter at the outer bearing journal AND the larger diameter bearings used.
 
What axle do you have now?

You can identify which axle you have by the tag attached to the inspection cover.  
 
Some feel better having disc brakes (I'll discuss this elsewhere:) and if you want discs then look for a Explorer '95-01, as these are the ones with the disks brakes. '91-94 axle assemblies are just as strong,  but have 10" drums (Which are more easily connected to your existing hydraulic lines and Parking brake cables)  

7.5-inch Rear Specifications 7.5-Inch Ring Gear 1.626" Pinion Stem

8.8-inch Rear Specifications 8.8-Inch Ring Gear 1.626" Pinion Stem

 

Swapping a Ranger 7.5-inch to a Ranger 8.8-inch is a direct replacement.  Swapping in a stronger 8.8-inch from an Explorer requires mounting the spring perches from under to over the axle.  This should be done by a reputable welder.  The '95 & newer Explorer 8.8-inch axles have disk brakes.  The master cylinders in the Rangers are not calibrated for rear disk brakes.  When converting to rear disks you should replace your master cylinder with one from a rear disk brake Explorer.

Axle Width Differences:

People have asked about the widths of axle shafts to swap shafts from one axle to another.

The early 1983-1992 Ranger axles are 1" shorter than the 1993-up pieces. (Both sides) meaning that the an entire Ranger axle assembly from 1993-up is 2-inches wider than 1983-1992.

The right side Bronco II axle shaft is 1-5/8" wider than the 1983-1992 Ranger shaft. All of this additional width is in the passenger side. The drivers side shaft is the same length in either axle assembly.

On ALL Ranger/bronco II's the drivers side axle shaft is longer than the passenger side.

There is no difference between 7.5 & 8.8 axle shaft lengths provided you stay in the same year window.

As a note, if you go to a salvage yard, you'll find many left-side shafts removed from 1983-1992 Rangers and Bronco II's. These axle shafts are the same length as the axle shafts in the "Fox" (1979-1993) Mustangs EXCEPT that Ranger/bronco II shafts are five-lug and Mustangs are four lug. Ranger/Bronco II shafts are frequently "pillaged" for use in converting a Mustang to five-lug wheels.

C-Clips:

The 7.5-inch and 8.8-inch axles retain the axles using C-Clips. A broken C-Clip axle will slide out of the axle tube.  C-Clip eliminators are available which hold the axle in the axle tube.

Ford Part#  M-4220-A

Jegs Part#  873-A1092

Axle Girdle:

Serious Off-Roaders should consider installing an Axle Girdle. They Replace the existing inspection cover and provide support to the differential bearing caps.  They also increase the fluid capacity.

Ford Part#  M-4033-G (8.8-inch)

Ford Part#  M-4033-J  (7.5-inch)

Jegs Part#   6-9-101-8.8 (8.8-inch)

Axle Codes:       

Rear axle codes can be found on the Safety Standard Certification label attached to the left drivers side door lock post.  The code appears as a number or letter/number combination in the 'Axle' column of the label.  The axle can also be identified by the tag on the axle differential cover bolt.  See the diagram above to learn how to read the tag.

Lockers:

A trucks off-road performance can be greatly enhanced using a locker.  There are different ways that you can lock the rear end. 

(1) An air locker (Expensive) solidly locks the rear end together using a small air-compressor attached to a locker in the differential. 

(2) A spool (Not Recommended) solidly locks the rear axles but should only be used for off-road competition since it doesn't let the axles turn at different speeds while cornering. 

(3) A Lincoln Locker (Same as spool) is simply welding the spider gears in the differential together. 

(4) A Detroit locker replaces the whole carrier and unlocks when turning. 

(5) A Lock-Right is the most popular because it replaces the existing spider gears, unlocks while turning, is affordable, and doesn't require setting up the ring and pinion like a Detroit Locker would.

Axle Truss:

An Axle Truss should be considered to protect your Axle from bending during serious off-road use, particularly if you like doing something silly like "Jumping" your truck. Desert racers in particular should have one of these because getting airborn is "normal use" for them.  For most people the additional strength added by a truss is cheap insurance against damaging an expensive axle A slight bend in an axle tube will quickly wear out the axles shafts and bearings may damage the differential itself as well as cause distinctive wear to your expensive offroad tires. And of course serious bending of the housing can cause the axle to break the axle leaving you stuck and in possession of a hefty towing bill.  

The only downside to an axle truss is that it will reduce clearance between the ground and the axle, but the clearance reduction is well worth the protection from bending an expensive axle assembly.
 
The only commercially made axle truss for Rangers and Bronco2's that I am aware of is made by James Duff and can be seen in their online catalog. 

Gear Ratios:

If your considering a change in your gear ratio you should use the 'Off-Road Calculators' on the main page of this website.

Ford 9-inch:

If you really want a 9-inch rear axle you can swap in one from a late 1970's Lincoln Towncar which has a width of 57-inches. The spring pads will have to be relocated to the top of the axle and the driveshaft will need modified to accept the larger U-Joint. Currie Enterprises (714-528-6957) will custom make a Ford 9-inch to your specifications.