Sounds like the brakes- if it was the transmission it would be both wheels. The brake expander plates are known for sticking on, or it could be that the brake linings are rusted to the expander plates or the axle housing. I would loosen the adjuster bolt on the axle housing (faces forward towards running board) and try "rocking" the tractor forward and backwards. If that won't work then the axle housing needs to come off- that could be a pain if the brake linings are rusted to the pressure plates. Maybe trying to pull the rubber boot off the brake adjuster rod, and spraying some penetrating oil in there would loosen stuff up- but it would ruin the brake discs. Sounds like they need to be replaced anyways though.
Good Luck, Richard