I just changed a set of these shifer keys on a MTD mower with a peerless transmission this past weekend. The problem was that it would not propel the tractor consistently, and slip. I got my parts from a Cub Cadet dealer since they use the same parts. The keys were about $12.70 each, and I changed them both.
After you get the transmission off the tractor, I would remove the bolts holding the case together and separate it UPSIDE DOWN, since the gears and shafts remain with the top side of the housing, at least that's how my model is made.
You will need to remove both shafts and sets of gears together. Watch out for that shifter ball and spring that gives you the click and detent feel when you shift gears. I had to move the shifter lever just right so I could finagle the shafts out of the housing. After you have both shafts and gear sets on the table, remove 34, 29A, 7 and 9. Remove the the old keys and replace with new ones. I had two extra hands for this part, but you may be able to do it alone. Re-assemble reverse of dissasembly.
This was my first time inside a transmssion, so I am sure if I did it, you can too. Good Luck.
By the way, keep plenty of rags on hand since this is a very greasy job.