After checking the inching valve if this makes no difference check the following below.

On the side of the transmission valve chest (end nearest to the front of the truck)there are 2x blanking plugs.Remove these plugs.Internally there is a port which you can remove with an allen key.This usually blocks if there is any contamination and causes the truck to lose all drive.I would also reccomend removing the transmission oil screen which is located on the underside of the left hand side of the transmission and has a small hose running to the transmission pump.We have seen a few of these where they are heavily contaminated with aluminium after the torque convertor has started to break up leading to contamination of the transmission oil system.