Try coasting with the trans in neutral and the engine stopped, or get somebody to tow the truck with a chain to see if the engine is actually the problem. It sounds more like you're hearing a brake, U joint, or rear end bearing noise.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!