I bet the hubs are not put together correctly. Remove the cover and take a peek. They are a little tought to understand the first time (mostly the spring and prawl setup) but once you have done it once, its darn easy.
Here is the best picture by pictures writeup of rebuilding aisins that I have found. With it, the newest of noobs can rebuild these things.
http://forum.ih8mud.com/79-95-toyota...b-rebuild.html
I bet if you jacked up the back of your truck, locked the hubs and put it in 4wd. You will be able to see the cv joint going to that "tough" hub spinning. It will be the easiest axle to spin so all the power in the front will be sent there.