MTD Yard-Man transmission problems

I have a Yard-Man Model 970-979. This mower is a walk behind with a 6 speed transmission. I'm sure you've seen these on Toro's, and other MTD manufactured mowers. Basically a belt controls the speed of the transmission which drives the rear wheels. When you push the mower manually you can hear a clicking noise coming from the ratchet mechanisms in the wheels.
Anyhow, after about a years worth of use (it is about 4 years old now) I could no longer pull the mower backward without having the drive wheels lock in place; almost like the transmission was still engaged. I've torn apart everything I could think of and cleaned it/lubed it to no avail. Once in a great while I can get the wheels to turn backward but it is a real struggle.
What am I missing? At least I have forward motion but it is a real pain not being able to pull the mower back with locked wheels!
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