My '97 Suburban has done this in the past, but I can't seem to remember what the solution was. When the car is in park, you have to depress the brake to get the car to go into gear. While we are pressing the brake, it is not allowing us to shift into gear. We have taken it to our mechanic, and he thinks the transmission is "getting in a bind" when you park on a hill. He recommended we just use the parking brake prior to shifting into park. The problem is that it still does it when we are on level ground. Any suggestions?