When you removed the seat, did you check whether it is the valve or the lever linkage that is stiff? If the valve rotates too stiff, you have to lube the input shaft with some thin penetrating oil. You may have to actually disassemble the valve body carefully, trying not to damage the o-ring seals. I've heard of a couple that got gummed up. On the bottom of the valve are two tubes going into the PTO clutch below, so if you can do all this without taking the valve out, it would help insure something else doesn't get messed up.
If it's the lever and lever pivot that are the problem, you have to take out the bolt on each end of the pivot shaft and remove it from the three levers that use this shaft for a pivot. I would remove the grey plastic side panel to make this job easier. Once the shaft is removed, it can be cleaned of dirt and corrosion and then lubed and reassembled. All three levers should work easier after this: HST Range lever, PTO lever, and FWD lever.