Check fluid level. Change fluid.

 

This ^^^. Open center hub cap , drain fluid and wipe clean, tighten cap back in place and replace the fluid with either Polaris Demand Drive Fluid or if you don't have any handy and want to try it sooner ATF Type F fluid 3.2oz in each hub. Rotate hub until the plug screw is at three o'clock, fill fluid until it starts to spill out, plug screw and rotate tire a few times, go back to three o'clock open plug screw and slightly turn hub toward four o'clock, let the little excess spill out until it stops, then rotate the hub back to three o'clock or less and tighten the plug.

Rinse and repeat for the other side.

 

The front wheel drive is not supposed work in reverse - I would suspect someone got the left hub on the right side and vice versa

 

Huh??

 

I stand corrected - the clutch in each front wheel hub is not directional - if it locks one way, it should lock the other way, but there might be a short delay when changing direction

 

Changing the hub fluid is always a good idea and should be done on a fairly regular basis to keep them working properly.
However, in your case, I'd first check the gear selector switch at the shifter box and/or the wires and connections running to it. (#'s 15 and 16 in the diagram) If the wires are broken or connections corroded or the switch is bad so that you're not getting a signal that the machine is IN gear the AWD won't work.
When you have AWD in reverse are you holding the reverse override button? If so, have you tried holding the button in forward gears to see if that will make it work?
Here's a link to one on ebay...they ain't cheap! I haven't done it myself but it should be pretty easy to jump it to override it unless you want to put it back original.

https://www.ebay.com/itm/Polaris-19...3233371-New/201487277122?epid=1423034028&hash=item2ee993dc42:g:w~sAAOSwCQNWdIIR