If you disconnect the speedometer cable at the transmission and look at the end of the gear that drives the cable, it should be turning clockwise as the wheels turn in a forward direction. Jack up one rear wheel and either have someone spin the wheel by hand or let the engine idle in 1st. gear. If the gear is turning the proper direction, try the same check by disconnecting the speedometer cable inside the cab. There might be a speed change gearbox somewhere between the transmission and the dash that is assembled incorrectly.