Transmission shifting to overdrive

I would not jump to that conclusion about needing a rebuild. I used to rebuild transmissions at dealerships and am certified in over 40 makes and models of automatic transmissions. It could be so many things that are serviceable, such as old dirty and thick fluid, varnished shift valves that can be cleaned up with an additive ( I have done it hundreds of times ), a worn and weak shift solenoid, a dirty and restricted filter etc, etc. Have an ATARCO certified transmission shop tech inspect your car before possibly, you throw out the 'baby with the bath water'.

Your transmission needs to be replace or rebuilt