2004 MTD model 609 shift problem

I have a MTD model 609 from Tractor supply that was slipping while in gear, belts were old and wore out/ Put new correct number belts on upper and lower transmission drive. Now when in gear the only way I can get it to stop or shift is to turn off the key killing the engine. The lower belt has play in it in neutral the upper belt is tight. Runs fine whem]n in gear just can't shift while running. Need help