I have a 99 Jeep Grand Cherokee Limited. ?

I have a 99 Jeep Grand Cherokee Limited. I have experienced 2x now a very loud clunking noise when I have been driving. Both times this has happened I have only been driving about 15 minutes. Then have stopped shut off the engine and came back out to my jeep to drive again after about 5 minutes. It immediately starts clunking while I am in reverse and continues clunking while in drive as well. Both times I have pulled back into my driveway shut off my jeep and after about 1 minute my jeep lurches backwards. After this happens and I restart and begin to drive again the clunking noise goes away. Any help on this will be greatly appreciated. I am wondering if it is safe to drive and what it could possibly be. Thank you!