Could possibly be the range sensor is dirty. HERE
is a post on how to find and clean it. If it gets dirty enough the dash lights wont match what position the lever is actually in.
If you do get a fluid change just have it drained and filled or have the pans dropped, filter cleaned/replaced, then refilled. It will only replace part of the fluid at a time. Do not get a flush. That will knock loose all kinds of junk that will plug up the tiny ports in the trans and kill it.
Edited by: Gizmo42