You are describing a surging which would be exaggerated in the lower gears if it were engine related.
From a cold start, drive about 1 mile using the brakes a minimum or preferably, not at all.Then immediately go around feeling the brake DRUMS comparing the temperature--be sure to include the parking brake if it is on the rear of the transmission.We are looking for a difference in one indicating the drum is not round or not fully releasing.
While driving at a speed and load where the surging is very noticable take the truck out of gear and coast.The surge should stop when the engine is disconnected if it is the source.
Give us some "feel" we can use.
