The first thing I would do is remove the right fender and disconnect the linkage at the pivot point between the pump and the control lever. With the engine OFF move the control levers and see if the stifness is still present. If the linkage is free than the problem is probably with the servo controller on the hydrastat pump. If the linkage is still tight than you will have to look at the individual linkage parts to find the binding. With the linkage disconnected you can move the pump lever up and down to see if you feel a tight spot in the servo controller.
