Transmission problem - 1994 L400 SpaceGear

[northernbcguy]

Having some issue with the automatic transmission. I noticed that the 'OD OFF' light stays on, hit the button at end of gear shifter and stays on?

The 'A/T' light is not always on, but once it heats up and drives. Goes from gear 1->2->3 then goes limp, kind of slips out of gear. I can then gear down manually to 1 (or 'L' on dash) and it engages again, and will gear up again.

Even if OD is OFF, it shouldn't slip out of gear I assume. Lots of reading thus far, and here is what I've found and been told:
1) the 'O/D Solenoid' - which I can't quite seem to find, there is a 4x4 solenoid, but don't think that will resolve gear shifting
2) transmission temp sensor - ordered already, not sure where it is located but will take closer look once part arrives
3) transmission ECU - bottom under the dash? My dash is off at the moment, so was thinking of replacing, unless there is a way to test it is working before ordering a new one?
4) OD switch - Unsure if there is a switch in the gear shifter or steering column that can be changed?

I also don't have a in depth shop manual (in English) currently. Any help on those items, photos, location of items, where to purchase items, how to test items, where to buy manual, etc. would all be very appreciated.