When put in reverse - vehicle suddently stops!

Okay - it happened again, and this time, based on your comments, I tried something different. This time, after the car abruptly came to a stop in reverse, I left the car in reverse, put on the parking brake and then released the parking brake. When I hit the gas, the car reversed normally. So, now the mechanic is rechecking the braking system and parking break to see if it could be a bushing or damaged control arm or something. Can you provide any advice that I could pass on to him? Thank you

I have 2001 ML320. Same exact thing is happening to me. Could it be transmission, or brakes?