I think it'd be pretty easy to figure out if it was the clutch or not. If you're running 15mph with the engine screaming and the clutch was the slip point, that would be generating a lot of heat and you'd probably be burning your bell, and it would get discolored. If this isn't the case, i really think its something in your transmission. The diff may be tight when you spin the wheels off the ground, but it could still be slipping. I'd pull the whole tranny out and rebuild it. If you've got new balls and rings in the diff and they still look fine, then i guess put everything back together, just make sure you didn't do something backwards. We all make mistakes no matter how many times we've done something What someone suggested earlier about holding the truck on carpet and trying to turn the spur gear can also help rule out some things. If you can turn the spur with the wheels locked then it isn't the clutch, its either the slipper or the tranny. Even if you think you have your slipper locked it could still be slipping if everything isn't in its right place. I'd pull that apart too and have a look at it. Good luck!