Where is it leaking? If the leak is coming from the ball pivot at the back of the transmission there are a few gaskets and seals that are fairly simple to replace, and not that expensive, as I recall. The majority of your time will be spent getting everything cleaned up and rust-free so the new seals won't be damaged during assembly or on the first test drive. One very important adjustment is the fit of the pivot ball to the transmission housing, which is adjusted by adding or removing shim gaskets under the ball retainer collar. To do this properly the ball must be removed from the torque tube so you can pivot it around and feel the drag on the ball. Once that's done, put a new seal under the big gland nut at the rear of the ball and reassemble everything. Changing the U joint is a good idea, since it must be disconnected to get the ball off the torque tube.
If the leak is back at the rear end, a lot more work is involved, and that does require removing and disassembling the axle assembly. An occasional pressure wash and topping off the gear lube is the simplest, and least expensive way to deal with the problem. You can buy several barrels of gear lube and a pretty good pressure washer for the cost of being anal-retentive about a few drops of oil. A bad internal seal at the front of the torque tube can result in the transmission oil leaking into the rear end and overfilling it. That can result in leaky wheel bearing seals, and eventually lube coming out of the axle tube vent.
Jerry
