What wrong with the rear diff the thrust bearing should cope with 80 90 plus runs before needing to be replaced.
If the diff is gritty you need to sand the groove out of the diff rings with 800 grit wet and dry until you get a shiny flat surface. Then bed it in by running it slightly loose under load for 20 sec retighten and it should be smooth a silk with the wheels on when tighten so it dosen't slip.
Over tightening a without bedding in a new groove will flatten the balls and cause that gritty feeling.
You should also run as trf boy says ceramic balls ($10 form acer)in the diff and only put smear of lube on each ball. This stop the diff slipping and attracting dirt so you don't need to over tighten it.
Also clean your hands and every thing else you are using to put the diff back together with any grit in the rebuild will make it feel rough.
And make sure the thrust bearing has being fully flushed out and dryed before relubeing.
better instructions click here
We have to do this for a 1/12th and 2wd drive off road as a bad diff can chronicly screw up the handling making the car lose it smoothness in the corners.