firstly i would make sure the brakes are actually releasing completely
check the park brake tension and make sure it is releasing
check the wheels, jack it up by putting chock blocks underneath the mast channels and tilting forward so it will lift the drive wheels off the ground and see how it operates.
make sure the lift is stable before starting work after elevating it

try deadjusting the wheels and see if this problem changes
you might have problems inside the wheels like loose backing plates or broken hardware inside the wheel hub(s).

if you do not find any problems in those areas then look for a transmission control valve problem or an internal problem inside the transmission.