420 Rancher drive shaft turns, but...

Lol it just sounds abused because it takes a lot to wreck splines in the rearend plus you rebuilt the top end. Debris is what causes the most havoc in rearend it gets past seals and ruins bearings. if you have any play it the diff with the axle in you'd need bearings as well. But I tell ya parts for a diff with labor is really high. Check the price of a new diff on partzilla to compare.
Speaking of debris did you find out why the top end went bad? Some causes are that it was hot then was ridden in water causing the cylinder to egg shape or debris(muddy water) got inside the engine or an air filter was either absent or super dirty or my personal favorite is when the oil plug on the bottom of the motor gets stripped leaving pieces of metal in the engine to cause havoc...... debris in the motor regardless of muddy water or metal pieces require bottom end work to remove or the top end will just go bad again.

It'd be worth checking those clutches first since the rear has problems if you have 4x4. Also listen for the oneway bearing going bad. When you kill the running engine do you hear a winding noise that comes to a clunk stop?