99 wrangler rear end

I have a 99 wrangler sport i bought about a year ago. It started making a knocking type sound about 6 months ago. It was real light at first, but seems to be getting worse. It only happens when i take off from a stop. It happens in both foward and reverse. It kind of sounds like hitting the inside of a plastic bucket with a stick really fast. At times there is a slight shaking, but not everytime. When the jeep is still, i can rock it and hear the noise coming from the left rear tire, like inside the hub. Any suggestions would be greatly appreciated. Thanks.

A guy I know had a broken stud in his back wheel....try taking off the wheel and looking at the studs. (Might just be a loose wheel nut).

Or...check the U-joints, they could be having a little, er, "difficulty"

Good luck