The text below is from Big Dean's original post (which can be found in the archives of this discussion fourm).
About a month ago I got my 135 stuck in gear. I was unable to resolve the problem following the advice of Big Dean. I ended up having to take my tractor down to the MF dealership--and they also weren't able to get it unstuck using the "ol' screwdriver trick" suggested by Big Dean. The MF mechanic was very surprised that he was not able to resolve the problem using the technique. He ended up having to pull the top plate off of the tranmission housing--which cost me $116.00.
Lesson learned: SHIFT SLOWLY USING THE PALM OF YOUR HAND (Instead of grabbing the shift knob and pulling up as you shift).
Big Dean's "Screwdriver Trick"
GETTING THE SHIFT LEVER UNHUNG
Pull your filler plug for the transmission look inside you will see a big gear. Find a level spot shut off the tractor. Push the clutch down & put the high low shifter in neutral to relieve any pressure that might be on the gears. With a long screwdriver or a small pry bar pry the big gear forward if it is hung in reverse. Pry it back if it is hung in first gear. If you happen to go to far it will go in the other gear. You need to get it in the center. You should be able to feel the detent balls pop in the grooves in the shift rail as you move the gear. Through the years I have probably unstuck at least a 1000 of them this way. It is a common problem on the Massey tractors after they get a little wear on them.
CAUSE FOR SHIFT LEVER HANK UP
The transmission main shaft lever hanging up is very common on most models of the MF tractors. The shift lever is held down by a spring & when you start to shift it you will unconsciously lift upon the lever. This causes the shift lever to go into the other shift fork on the other shift rail. That rail can not be moved because of the lock on the back of the transmission to prevent it from going into two gears at the same time. Once you have both shift rails in the neutral position you can
then move the shift lever.If you will use the palm of your hand When changing gears to push the lever into neutral then get hold of the lever
to put it in the gear you want it in. Shifting it that way it will not hang on you again. Below is the article from my web site that tells how to
get the lever unhung. If I can be of further help Email me at firstname.lastname@example.org.