Couple of things to look for. One is the backlash. Did you check that when you had the cover off? Should be somewhere in the .005 to .008 range. The other thing is the pinion seal. If the pinion seal doesn't get seated all the way down the pinion flange can hit it and cause a lot of drag. Had a problem with that one myself. Last thing would be the pinion bearing preload. If the guy who rebuilt it reshimmed the pinion preload he may have screwed it up somehow, like maybe he didn't put enough shims in. Usually if you're doing bearings you don't need to mess with the preload, but some people do.
FWIW, I took my first diff to a normal good ol boys American gear shop and they f'ed it up 3 times. There isn't that much to it, and I'm still not sure how they managed to screw it up so bad, but they did.
There is NO reason why you should need any sort of lever to turn the pinion. If that's the case, something is WRONG. Take it back and demand the guy fix it right.