Newbie with locked rear wheels on Sportsman 500
So I am pretty much brand new to ATVs, but have some mechanical skill. I bought a pretty well used 2005 Sportsman 500 HO last fall and rebuilt the top end of the motor. Used the machine through the winter plowing the driveway and some small jaunts around the neighborhood. Everything seemed to be going well, until just the other day. My son took it on a trail and was headed up a hill in LOW gear with the AWD turned on. Part way up the hill things just locked up and the wheeler would no longer move, found the wheels locked up. The shifter was also locked in LOW. Got the machine home and on jack stands so all 4 wheels are off the ground. Was able to work the shifter loose so it now goes through all the gear positions. Engine starts and runs fine. Looking at the belt from the large breather hole on top of the belt housing, the belt looks OK of what we can see of it. The rear wheels barely rotate forward or reverse before clunking to a stop, and the front wheels spin freely forward, but only barely spin backwards. Brakes are not locked. Best I can surmise by what I've read is I'm in for pulling the tranny and cracking it open. Ideas as to what may have failed?