XP 850,I know they have a recall for the vent,but iread something about rear drive or rear diff poping,what it is.

 

mine doing same thing>dealer says its normal but annoying. i heard if you complain to polaris they will replace the axles. they are using different axles now than they started with

 

The noise you are hearing the the axles clicking, if you take it in they should change them out under warranty (with some possible complaing, but none the less they should do it)

 

How common is this problem? Has it been resolved in the newer production runs? If so, when?

 

i am kind of confused with the problem. what actually happens?

 

Polaris knows about this problem and will replace the axles. They received a marginal batch of axles from a supplier after their first run of machine's. They are ok to use like that but annoying because of the popping or clicking noise in the rear.
Have your dealer call Polaris and find out about getting them replaced! You will need your VIN # and date of manufacture to give to Polaris. Good luck!