Start with the simple things first.
Replace the main hydraulic filter.
Check that the aux hydraulics are not stuck on, grab the right stick and pull it to the left and let it spring reture to centre. I don't think this is the issue, as it mainly messes with the tilt/lift system, not drive but check it anyway.
The next thing to check is the charge pressure, there will be a sender at the hydraulic filter housing. Install a manual gauge and check it, it should be around 110 PSI. Charge pressure is needed for the drive system.
If that still doesn't work, we can go deeper.