My 1997 12-valve, automatic transmission, truck with 187K miles sometimes will not shift into Drive or Reverse. It happends less often when the engine is fully warm. The shifter will move, but not the vehicle. Not other serious transmission issues. Is it likely that adjusting the linkage will help ? What about that Lucas additive ? What signals the need for replacing the transmission? Any recommendations for transmission repair in the Virginia Shenandoah Valley area ?