My 03 sportsman 600 only has 3wd. The front right tire doesn't turn when in 4wd regardless in forward or reverse. Everything on my speedo works except speedometer.

 

My praire 300 did the same thing unless I had equal bite on both front tires.
On many occasions I would find just one front wheel turning, however I soon learned that if I straightened out both front wheels and hit the throttle hard both would spin, but if one got more bite than the other, one would quit turning. Probably why many have the diff lock feature nowadays.

 

Does the left wheel grab? If it does then I'd check the right driveshaft. If it's ok then could be an internal diff problem. If the left wheel doesn't grab also or even if it does,you can also jump the brown/white wire in the speedo harness to the solid brown and see if you have power to both wheels. Easiest way to tell is to have all 4 tires off the ground. Dirt Cheap Yamaha, Polaris, Suzuki & Kawasaki OEM Parts & Accessories – Cycle Parts Warehouse Plus no speedometer is usually because of a faulty speed sensor located on the right front caliper.

 

The Polaris in a AWD system its in 2wd or true 4wd there is no in between 3wd like on a limited slip 3wd ATV. That's why Polaris has no need for a diff lock system. When the rear wheel slip both front wheels lock at once until the rear wheels regain traction and return to 2wd.

If you have the locking hub set up change the on demand fluid in both hubs. If you have the single locking centralized set up change the on demand in the front diff , or it may have the broken plastic roller cage issue.