42" 15.5 hp mtd yard machine will only move in 7th gear.

I have a 15.5 hp 42" cut mtd yard machine. The engine runs great. I was cutting grass in third gear when it just slowed to a complete stop. It would not move in any gear but 7th. I have changed the rear drive belt and the spring on the tension pull. I've also jacked it up and turned the transaxle pully by hand and the wheels turn fine. I'm not sure of the linkages and how all of the springs. If anyone has had this problem please help.