I have a 1996 Case 1840 skid steer, recently it quit pulling on the right side, I had noticed in the past that that side seemed to sorta lag back, but it still worked, anyways, it quit pulling altogether on that side, so I pulled the tandem pump out took it to a repair shop to see if that was the problem or the drive motor itself, he called said it was the pump, so after waiting about 3 months, I finally got it back. put it together & still no response, but also I have no response at all when I push both leavers forward at the same time, which I did before but the left side is the only one that would go. So could it still be the pump, or the motor. I'd think I should get some reaction when I push both leavers forward. it does act like it's wanting to go, but doesn't.
Low hydraulic fluid or air in the lines still.
Are you getting equal pressure to each drive motors? Whats you pressures? Do you hear anything coming from the side thats not moving? Is that side locked up or free wheeling?
Remove you hydraulic filter...cut open...check for metal....sounds like drive motor failure.
seen one that the bolts were out of the sprockets broke off after running loose acted like what you are dealing with but as MUDDOG1975 said--check for same pressures for both motors