Mustang 940 left wheels locked up

I have a older Mustang 940 that worked when parked. The left wheels locked up after sitting for about 4 months. The bucket/lift hydraulics work, and I can operate the right side wheels. When I try to force the left wheels to move by turning the T stick hard right and forward or back, it stalls out the motor. I looked into the hydraulic reservoir and found it empty, so added fluid...it leaks out. Initial research leads me to look into cracked line, or a relief valve. worst case is the drive itself. Could the relief valve be stuck open and how do I check? Looking for an easy way without calling in a pro.