I've re-built them FNR MTD craftsman types.
I usually have about 3 laying around for parts, take 3 and make one.
You can get the axle bushing and few other parts, but have to be careful or can get more money into one for parts instead of just buying a good used one.
If it's shifts ok with the engine killed do as the guys says.
If you don't like working on transaxles and don't have patience, look around and just replace with a good used unit. If it was whining at fast forward before it quit working it's most likely worn out inside. It was whinbing for lube because the old clay grease was hard and not lubing. (squeaky wheel get the grease) No way to add grease without opening up the unit and packing. I have added about 1/2 cup of 90-140 diff oil to them (with good results) by carefully drilling a hole in the center casting.
I refill (repack) them with EP2 grease and little bit of 90-140 oil. Re-pack the outer axle bushing area really good. The axles do not have seals for oil so just use little bit of the oil.
The existing grease is a CLAY and seems to be specially selected so as to allow the unit to wear out. (consumable) It will be hard packed and dry. It's also selected so as it won't leak out.