Overdrive on 96 Exp no longer works

96 explorer, not 4wd, 265K miles, tranny was overhauled abt 100K miles ago by Scamco (big mistake, but thats another story) . Started recently to suffer from torque shudder going from 4th to overdrive, and now overdrive never kicks in. The overdrive light doesn't come on but that hasn't worked in many years. Any help?