Are you sure it's a rear clunk or the transmission slamming into gear? I always dread parking on a steep incline without a working E-Brake.
Put it in neutral and move both drive shafts in every direction at the joint, any movement is a bad thing. Could also be the slip joint has no grease in it where the shaft enters the transfer case.
Being intermittent in general is strange if it's a moving/rotating part. I was just throwing out what could cause a vibration.
The drive shaft is closest to the floorboard at your feet, I would start there. Wheel bearings are easy to check for regardless.