ALSO:
Check the torque on the lugs.
Also, their is a Jesus allen screw holding the diff gear works together.
Some Rzr 4 owners are reporting that they never got torqued down properly causing the diff to grenade.
Evidently, judging by the pix and horror stories, they are a PITA to get to to check and tighten.
The front Diff on some RZR's uses a special fluid. Ya can't use the rear fluid or transmission or transfer case fluids or the front won't go into 4wd.
It's a super lite fluid only available from Polaris.