ok so i just installed a welded up diff into my 3rd and the movement between the pinion and the ring gear isnt super smooth just yet, but keep in mind it has no fluid in the axle this i noticed just trying to move the driveshaft myself, will the gear like "fix" itself eventually to where it gets smoother, like after it wears it in

 

New ring and pinion, or an old diff that is "newly" welded?

 

Sounds like you either got weld spatter on the gears, or your setup is off, every fresh third I've spun was as smooth as a good stocker.