That could be it...
Also, on Saturday, I backed up to the trailer and was hooking everything up, the truck had been running for about 5 minutes, and the tach was dead. A minute later I hear a mild lope, and the tach comes back to life, and then by itself, it revs a couple hundred RPM for a second, and then drops back down to normal. All the gauges looked fine, no smoke of any color, and the truck runs fine, just the CEL is on and the P0336 code. (The scan gauge shows 3 codes: P0121, which has been there for a long time, and then P0336. It won't show the 3rd code for some reason, but I'm thinking it's just P1693. It won't let me erase the codes either)
But after that, the tach works fine, and the truck runs like a scalded dog for 2 hrs, dragging a trailer 100 miles...