Even though you replaced diff, when i did mine, since it had been sitting for a while it was a bit stuck. BTW it did have new Redline 75w-90. I took it to a big empty parking lot and cut very slow donuts in each direction, and then figure 8's. A few minutes of that got it working right.
I'd also jack rear up, and turn each wheel and see if anything strange happens. Then lock one wheel in place and turn the other, it should still turn but it should be harder.