Sounds like trans is shot. If you have $30 to blow, you can always go to hardware store and get a 12 foot piece of 1/4 clear hard tubing and siphone the trans fluid through the dipstick into a LARGE pan (bucket won't work because bucket will fill to the lower part of the trans pan and quit draining). Measure the amount of fluid extracted and replace with DEXTRON VI from Wal Mart. Maybe something is stick and the new fluid will loosen it up. I doubt it, but it is an easy one time shot in the dark as opposed to dropping the trans.
2001 Bonneville SSEi, 219,000 miles.
Whats in my driveway:
1998 Chevy S10 4x4 ZR2, 1999 Olds Bravada, 2001 Pontiac Bonneville SSEi, 2007 Ford F-350 regular cab 2WD, 2008 BMW 335i coupe (Wife'* replacement for her Bonneville). The Bonneville is my favorite of all the cars in the driveway.