Mule 550 wont move (unless differential is locked)
 

I've got a 2004 mile 550. It will not move in forward or reverse. However it does drives fine in both directions with the differential locked. Any ideas?

Sounds like you have a sheared driveshaft on one wheel. It probably will be the splines in one hub gone, but could be the ones in the diff. Jack it up and see if you can turn a wheel by hand with the gears engaged and the diff locked. If one side turns easily, check if the shaft is moving with it.

I see I have 1 bad hub that's slipping. I'm trying to figure out which wheel pulls normally? I swapped sides with the hubs and it still does the same thing. Drives fine with diff locked, and wont fo with it unlocked. I'll order 1 new hub.

I repeat "Jack it up and see if you can turn a wheel by hand with the gears engaged and the diff locked. If one side turns easily, check if the shaft is moving with it." If you do this, it is the side that moves easily which has a fault, and it isn't always the hub, so find out what the problem is before buying spares.