I see you're new here, which is ok, but we do have sections for posting questions exactly like yours. Many have answers "stickied" at the top of the forum pages.
Such as this one in the "Ranger Axles" section.
http://www.therangerstation.com/forums/showthread.php?t=15
This covers front and rear, but you'll see it has things for beefier U-joints and manual hubs, which are on the DIY level (and really are the 2 biggest faults). Technically there's more with the shafts such as running a "Super 35" Jeep carrier and cutting down D44 TTB shafts or having ones made, but that is pretty expert territory and not something I recommend unless you know exactly what your doing. Most will argue this is not necessary as the u-joints usually let go on the stock shafts first anyways.
As for your center slip joint, unless the U-joint is loose in it's holes, slap a new joint in there (Spicer 5-760X is the better joint, you'll see in the article) and do the full circle clip mod and you'll be in business. Or are you saying the splines are shot and worn out? In which case, junkyard, grab the whole front axle.
Edit, thought of something after I posted.
Slip yoke:
http://broncograveyard.com/bronco/i-82086-d35-oe-right-slip-yoke-axle.html
(Says it fits 4.0L Ranger, but really any with a Dana 35)
And the corresponding shaft it mates with:
http://broncograveyard.com/bronco/i-82082-d35-ifs-right-intermediate-axle.html
I've been in their shop before, pretty cool guys.
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