The truck is fine...I believe this is operator error.
It won't work right with an empty truck driving on a flat road, your ground speed is to fast and there's no load on the rear. Find a long steep hill and see if that makes a difference. If the rear works in 2nd and 3rd than it will work in 4th...it doesn't know what gear the trans is in.
With my 2-speed rear trucks I drive them all the same...start in 2-LO, 3-LO, 4-Lo and then shift the rear to 4-HI. Split shifting is only needed when the truck is really laboring to get moving.
Mike B
