I have a 2001 trail boss, the rear sprocket has some play. It doesn't move side to side but forward and backwards. All of the bolts holding the sprocket on are tight. Without taking it apart I can't seem to figure out why this is happening. When you turn the sprocket the play appears to be between the part that holds the sprocket and the axle. The axle bearings seem fine since there is no play in them, only play is with the sprocket. Any thoughts!

 

It will be between the splines on the axle, and the splines on the sprocket carrier.
If you have a concentric drive machine, it is not uncommon for a little looseness to form. As long as it isn't bad, I wouldn't worry about it. My Xplorer has had that looseness for the las 4000 miles or so.