Tranny troubles; any advice?

I currently have a 1992 Chevrolet Lumina Euro, 3.1 engine and a 4-speed transmission with a mechanically-engaged overdrive. Recently, I have been having problems with my overdrive gear. When I go to work, which is about 45 miles away, the overdrive would not engage at all until about a mile away from work. I have already changed the transmission fluid and added some Lucas Oil transmission treatment and it still will not work. Any thoughts? Hopefully, this doesn't resort to me rebuilding the transmission after I get a newer car in better shape.