Hi,
I've lifted the top cover off the hydraulics of a a similar tractor. From that experience I would say that this issue could be either the shaft that the lever rotates or the internal linkages attached. The internal linkages are a complex set up that control the hydraulic fluid flow to perform the position and draft control actions. They can break or wear and interfere with each other, causing a jam. I suspect that the hydraulic top cover will need lifting to inspect.
Get your hands on a MF135 manual, and beware, if I recall correctly, MF35's have a different utilisation of similar hydraulic control levers, suggesting that the internal linkages will be different, so don't use a 35 manual.