2003 Pontiac Montana van won't shift into reverse

First of all, I sent this question through yesterday but I'm not sure it went through because I had made a contribution from PayPal that didn't show up on that account. Well, I'll try again.

I've been having trouble trying to shift from park into reverse when starting the engine. GOing into Drive or neutral is fine, but I can't back up. WHen I try this, it's almost like the emergency brake is on (even when I didn't set it). The van lurches back a few inches and then stops. The engine keeps running but I can't back up. If I shift into drive and then go forward a few inches, usually I could then shift into reverse and it would work, but even yesterday I was having trouble with this.

I've already had it to the mechanic once but they couldn't find anything wrong, but I'm not sure they understood the problem since if I shift into drive (and there's room to actually go forward), it'll work fine. But this doesn't happen all the time, and it's not limited to the engine being cold or warm. It can happen any time, but doesn't happen every time.

After about 10 minutes, I actually forced the van to go back far enough that I could roll into drive, and then promptly took it back to the mechanic, where it is now.

Any suggestions?