Well, my truck will live to tow another trailer.
The fault was with the brake pressure transducer. Had it replaced and at the shop they hooked up to a trailer so I could test it. Seems to work just fine now. I'll test it again with my own trailer on Monday because Thursday I hook up to a loaded steel gooseneck trailer for the haul into the mountains.
Thanks for everyones interest and input. I'll definitely use this site again.