When you say "stuck in overdrive" do you mean that the overdrive is engaged or do you mean when you pull the overdrive cable out the car does not return to conventional drive? If you pull the cable out and the car still freewheels (no engine braking when you let off the accelerator) at speeds lower than 29 mph then the problem is the cable likely is not pulling the lever on the transmission forward.
If what you're asking about is the overdrive remains constantly engaged, the quickest test is to see if the car will go into reverse. If it will, the overdrive is not engaged. If it won't go into reverse, turn off the engine and try to put it in reverse. If will go into reverse with the engine off (ignition switch in off position) but not when the engine is on, then you likely have a sticky relay. If the car will not go into reverse with the engine either off or on, then the solenoid is stuck.
Let us know if this helps.
