hey guys, I have had a really loud humming comming from my truck when I step on the gas.. if I let go it stops.. mostly everyone said it's my rear. Well that noise just got alot louder so yesterday I launch my jetski and as I'm about to back down the ramp I turned it into 4x4 just incase soemthing went wronge I would have front wheel drive to pull me up.. well as i'm in reverse in 4x4 I hear a loud CLUNCK so I moved about another foot and heard it again.. can't go in reverse in 4x4.. so I took it out of 4x4 and went in reverse fine (just with that humming noise). So my question is does anyoen have any idea's? Could it be the 4x4? rear bearings? Since i'm not sure what the problem is would it be easier to just buy a used rear and put it on? or just let a mechanic try and fix it.. I think that the mechanic might be alot more money trying to problem solve.. and I would have a friend put the used rear in for nothing. If I put a used rear in would it fix and 4x4 issues/bearings?

it's a 97, 4.6l 4x4 with 105k miles.

thanks for any input