My truck thumps? at 40 mph. The rpms do not change or jump, and it seems that the thumping does not occur while accelerating rapidly. However, it does happen consistently when hovering around 40 - 50 mph.
Where is the thump coming from? Does it occur under load? Have you tried putting it in neutral while at speed to see if the goes away?
Perhaps it could be a bad belt in one of the tires?
There is no load. It is coming from the engine, and makes the whole truck shudder/jump. It only happens once just as I accelerate past 40, and then it stops. But it will happen every time I pass 40 mph.