[What's_nitro?]

Here's a wierd question; Is it smoothly hard to turn, OR are there obvious "bumps" along the way that make it hard to turn? The former would suggest that you used a higher weight oil like George16 said, while the latter means you should re-check the diff shims. Also, if you are feeling this while it is installed in the truck you may want to check your driveshafts for binding. I've learned over the years that just because you put it back together the way it came apart doesn't mean it will work properly.