NitrousN, thank you very much! I have the shortest wheelbase 1500 for that year. This makes alot of sense, and is what I believe is happening to my truck.

Transman, this makes perfect sense, because overdrive engaged this afternoon after school. It operated smoothly and strongly for two hours, with no glitches. So the low temp thermistor must be exactly what was preventing my OD from engaging. Thank you so much for your help through all of this.