One thing to remember is that the drive shaft can act as a sound tunnel making rear end noises appear to be in the front and vis-verse.
Is there also a clunk when you get on and off the gas peddle. It could be as simple as a U-Joint or it could be wheel bearings or the gears(low on fluid). It could even be the tranny low on fluid or the fluid could be going bad if it hasn't been changed in a while.
Those are my guesses.