AutoTrans lever will not shift out of park

Yesterday I drove home (without issues) a 96 Sierra 1500 (acquired from an estate). No problems. It was parked on the street when I got into the car and there was no difficulty shifting into Drive and driving away. I drove it home and parked it in my inclined driveway (this may be an issue), set the parking brake and then shifted into Park. This morning I started the engine and, with my foot firmly on the brake pedal, attempted to shift into Reverse (or any other gear for that matter). There was essentially no resistance in moving the column shift lever down and up, but the gear indicator never moved from Park and the car will not move. I changed the engine oil and filter last night and checked the transmission fluid level this morning. Although it's not parked on a level surface, the transmission fluid level appears to be in the low end of "Cold" on the dipstick. Any suggestions?