"click click" in rear end upon moving

When I move from a stop in Drive or Reverse, there is a clicking noise coming from the rear end of the truck. Usually two clicks. Happens everytime I set off.
Have seen some posts about this but havent really seen a clear answer. Just recently purchased this truck used with 80k miles. New to owning a truck in general so im not familiar with servicing driveshafts or u joints...I did check fluid levels on both diffs already but i dont know where to start as of right now. Any help is appreciated!