1996 GMC Sierra 1500, 4.3L V6, AutoTrans, 2WD, 190K Miles

Acquired from an estate yesterday. It was parked on the street when I got into the truck, started the engine and shifted into drive. Drove it home with no issues at all. Parked it in my inclined driveway (may be an issue) and, with my foot on the brake, I set the parking brake and then shifted into Park. Changed the oil and filter. This morning I started the engine and, with my foot firmly on the brake pedal, tried to shift into Reverse. The shift lever showed essentially no resistance moving down and up, but the shift indicator never moved out of Park. I checked the transmission fluid, not on a level surface, and the level is in the lower portion of the “Cold” area on the dipstick. Any suggestions as to what the problem may be in getting the transmission to go into gear?