If the gears shift easily with the engine off, but are difficult when the engine is running, the belt is keeping tension on the transaxle input pulley. This is usually because the belt keeper (or one belt keeper) is adjusted too close to the belt. The belt keepers should be about 1/2 inch away from the back of the belt when the belt is tight.
If the transaxle is hard to shift with the engine running or turned off, the problem is inside the transaxle. The transaxle has betadine grease in it, not oil. The transaxle would need to be taken apart and cleaned and the shafts and gears coated with a coat of good quality grease. At this time you should check for wear.
You can also replace the transaxle with part number 170268. The cost is around $180.00.