Ok I am thinking it make be that the brakes are locked up as the only wheel that is turning is the rear left- the only one that is not braked.
But if that were the case wouldn't the wheels still be locked up when I jack it up off the ground?
RayCole, seems like a few different things might be going on here. First of all, definitely check to make sure all three brakes are not sticking causing your "locked up" situation. Once the machine is rolling freely you'll be better able to diagnose it.
Secondly, check all your fuses (under the seat behind the battery) to make sure that they are all still good. If they are, you may be chasing some electric shift issues. If the motor turns over when you press the start button, then I'd say your neutral switch and the display are good, because the 2001 Ranchers did not have a start in gear feature, they only start in neutral. If you're getting no spark in Neutral, check out this.
Lastly, if it truly is the left REAR wheel that is rolling while the other rear wheel is locked up, then you have either a stripped hub or a broken rear axle. The reason there is only one rear brake is because the rear axle connects the left and right rear wheels together so they always receive equal drive (and braking) force. Under normal operation, the rear wheels cannot spin at different speeds. So if you're getting rotation out of one and not the other, something isn't right.