All "RE" transmissions have the temp sensor (thermistor) in the wiring for the OD and LU solenoid - its inside the trans where it see trans fluid temperature. The earlier OD units had the external sensor in the front cooler line - OD is disabled when the fluid temp gets around 250-265 F. The O2 splice can cause this issue as it cause noise in the circuit. In order to get OD you have to be in 3rd gear (except 48RE); have to have a stable TPS voltage at less than WOT; warmed up but not at the disable temperature noted above; neutral safety switch has to report NO Park or Neutral position; ambient air temp above some number I cannot remember anymore (-5?); speed sensor reporting higher than 35 ( I think), and I may have forgotten some others. Try setting cruise control - if it doesn't work then look at the neutral safety switch, TPS, clock spring in the steering wheel (rarely an issue but has been a cause). To check the TPS the best way is an analog voltmeter - the swing of the needle must be smooth from closed to WOT - if the needle bounces around you will have an issue. This is reallly common on the diesels - they kill the factory TPS.