Driveline and gear noises will transmit up and down the length of the truck and appear to be coming from places they aren't. Put the rear end up on some heavy duty jack stands, chock the front wheels securely, and roll under with a stethoscope or a broomstick to check out possible noise sources. 4th. gear might eliminate noises from inside the transmission, as there's no gear reduction going on other than the rear end, or it might just be speed-related.
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!