Jack and block the drive off the ground. have someone stand on the brake pedal and do a visual to see if the brake assy is working correctly and releasing brake. If brake is working correctly, with it released see if you can manually turn the drive wheel. These trucks are common for drive unit bearing failure. You can also tell a lot by draining the gear lube into a clean drain pan and inspecting for metal.

If it comes down to pulling and replacing/rebuilding the drive unit... I suggest (if you are unfamiliar with lift trucks) getting help from a trained tech. This is a mast up job and a lot of safety steps come into play to keep you from getting squished/killed.