Assuming you've checked and found the trans fluid is properly full, the usual "failure" mode of these O/D's is that the electrical stuff has a problem...worth checking before the work and expense of replacing O/D. Solenoid, relay and wiring seem to be common problem areas.

With ignition on (engine not running) and shift lever in 4th, get someone to trigger the O/D switch on and off while you have your head under the car and as close to the O/D as you can get and listen for the solenoid - should give a healthy "Click". If it clicks - unfortunately the problem is likely inside the O/D. If not, step two is to bring a wire from the battery positive post or fuse box (carefully!) and touch it to the solenoid positive terminal...(the negative terminal carries a short wire grounded to the O/D case). If you don't get a click: bad solenoid, replace it before going further. If it clicks - bad wiring or relay. Post back with results.