Hy Debbie Jo, your problem could be as simple as a fluid and filter change, or as bad as a transmission rebuild. The 4L80E is a heavy duty electronically controlled transmission, the E.C.U. [computer] has to be able to communicate with the engine E.C.M. [computer] and all of its sensors, or problems like this may occur. I would take the vehicle to a shop that can download any codes from either computer, if there are no codes then I would take the vehicle to a transmission shop you trust and have them diagnose it, hope that helps.
