1beastlydude 2/1/2013check your u joints
put one side of the rear axel up on a jackstand, make sure the truck is stationary, then put it in nuetral
get under it and try moveing the shaft by hand up , down ;and side to side, then rotate it by hand.
it the u joint is bad you will see play and hear noises.
However if it isnt the u joint , at least you can investigate other things while you are down there.
the noise could be from the differential, have you ever checked the gear oil?