My standard disclaimer: I provide
legal information on the forum.
Your owner's manual limits the '85 Ranger to
2,200lbs trailer towing for the 2.8/manual/4x2.. (It's really 5,000lbs GCWR, as the 4x4 tows 200lbs less) See the attached Towing guide - its a copy of a copy but the best I have.
Note: The 2,200lbs is based on the truck having 3 - 150lb occupants* in the cab and full tank of gas, no junk stuff behind the seat/in the box, etc. The 3 occupants is derived from the number of seatbelts, 150lbs was (still is for J2807 trailer standard) the average weight for an individual. So, in theory you could tow a little more than the manual, legally. Just no sandwich maker in passenger's seat(s), etc...
Arizona trailer law requires truck with trailer having less than 3k lbs. GVW to be able to stop within 40'; a number of other states (Pennsylvania, Washington) add a caveat; the trailer needs brakes if >40% of the GVW of the towing vehicle ('85 Rangers start at 3,800lbs GVW = 1,750 lb. GVW trailer).
The fine print: The trailer weight is the GVW of the trailer, not the actual weight of the trailer. (There is an assumption that you might fill up to limit but not overload the trailer.)
To address some points above:
A Cologne V-6 (2.8) is a Cologne V-6 (2.9) is a Cologne V-6 (4.0 OHV) is a Cologne V-6 (4.0 SOHC) as far as mating transmissions goes (OK, there is an extra bolt hole for the SOHC, but it isn't critical for this discussion). Therefore it would be possible to replace the weak Toyo Koygo with the stronger Mazda M5ODR-1.
It is possible to upgrade brakes on '85 Ranger:
Replacing knuckles with '95+ to get larger rotors/2 piston calipers)
Upgrading to 10" drums/Aerostar drums (finned) if you only have 9" ('85 will have smaller axle hubs, so Aerostar drums should be a simple install.)
The above don't change things in the eyes of insurance, but they help with peace of mind.
Trailer brake controller (The state of the art ones with accelerometers are pretty hands free)
To stay under 2,200lbs, tent trailers like
@85_Ranger4x4's are your best bet. (There might be older Scamp trailers in that weight range too.)
p.s. The trailer sales guys will sell you way too much trailer given the chance - bigger trailers cost more = higher commission.