there's a lockout device in the trans that operates under pressure. once the trans gets moving, say, 5 mph or so, pressure pops out this rod that blocks the shifter so you can't go into reverse at too fast a speed.

pull the trans pan. assuming your feet are hanging out the drivers side, the valve is on your right, in the drivers side, front corner on the side of the valve body. i forget how many bolts, but it's no big deal. clean the sludge out of the valve, make sure it's nice and free and reinstall. btw, changing the atf every 30k, like mb says, USUALLY prevents this. i've only seen it on cars that have not been serviced. good luck, chuck.