You need to start with a visual inspection. Safety first, so properly block the wehicle and start looking for loose parts or slop in the system. What I mean is you cant check for a loose ujoint or pinion if the system has tension on it. If its on an incline in park or the parking brake is on the rear drivetrain will have tension on it. Block the vehicle so it dosent roll down the driveway and go under and start checking things for looseness. Could be a bad ujoint, loose pinion nut, slop in the drive gears, loose wheel, spring bolts,bad bearings, loose splines, etc. You might also get the drive wheels off the ground (on jack stands) and have someone hold the brake and go from forward to reverse and you check for movement or looseness. If you dont do these things porperly or safely you could be run over or crushed or sucked into a spinning wheel. I dont want to preach or sound like a hard ass, but if you cant do it safely take it to a shop and have them look at it. Dont let the first guy that looks at it tear into it. Thats a starting point. Search the site and there are lots of experienced people on here that will probably reply. Good luck.