As long as the electrical control side of the drive system is working correctly it sounds like a bypassing drive unit. To check for a bad unit remove the case drain line inbetween the two main drive hoses on the back of the motor. Run a new line from the drive units case drain into a bucket. Measure amount of fluid coming out of the case drain line. If its more than 1L every couple mins then the unit is bypassing internally. Repeat same procedure for the other drive units on the truck.
Best to do so when the truck isnt moving when a drive direction is selected.
Keep in mind the unit may not bypass if the wheel is lifted off the ground.