Here's an experiment to get more info. Note I haven't done this myself--at least not lately--so if anyone sees a flaw in my logic pipe up:
Get on a straight, traffic-free piece of road. Get into 4th/overdrive in the normal way at about 50-60MPH. Put in the clutch, put the gearbox in neutral, flip the O/D dash switch to 'Normal' (off) then rev the engine. This should isolate all the electricals from the O/D. Then put the gearbox back into fourth and let the clutch out gently (give it a little gas if necessary); if you're still in O/D as evidenced by engine RPM vs. speed the problem isn't in the electricals (it isn't likely, but not impossible that the solenoid is stuck in the on/up position). If you're still in O/D the problem is in the O/D itself: either the operating valve is stuck open or, as John suggested, the O/D cone clutch is embedded too far into the brake ring (that's all I can think of, anyway).
Have you tried fresh fluid? If the operating valve is sticking it might free it up. How many miles on your car? Has its fluid been changed regularly (20K miles or so for dino fluid; synthetic can go longer). I pulled my O/D at 205K miles and the cone clutch lining was still serviceable.