[quicksilver97ta]

How sure are you that the tires are not loosing traction? Just because they dont leave a striking black line or the typical noises doesn't mean they aren't spinning. There is a wide range of what kind of traction you get with different tires and their age plays a big factor as well. But if they are not spinning take a really good look at where the rear mounts onto the leaf springs. Look for any signs that the rear is moving around on the springs and the condition of the rubber perch pads. An upgrade to the stock configuration is to use 2 sets of u-bolts on each perch instead of 1 set u-bolts and one set of t-bolts.
As far as a rebuild, in order of priority, change the fluid, replace leaking seals, replace axle bearings, replace all the seals. Thats about it unless it's making noise.