Well, after being out of commission for a week with the flu (working in cold garage late at night whilst also having a cold = getting the flu) I've just now gotten a chance to put that timing light on it. It's waaay off of 15 degrees BTDC, closer to 25 degrees ATDC, when it's running it's best. If I turn the distributor back to 15 d BTDC, the engine nearly dies. This makes me think that smj99smj may well be right in that my belt is off by one tooth. I will be checking the computer codes, however, before cracking into the timing belt task.
Biggest challenge I anticipate in checking belt is possible need to remove the main pulley. I don't have an air ratchet hammer (if that's the correct name), and though the electric one I rented to loosen the main bolt initially worked very well, it's large size does require dismantling the front of the truck (coolers, grill, bumper, etc). Hopefully I can simply remove the top half of the belt dust cover, allowing the belt to stay wrapped around the lower main gear while I slip it off either of the top cam gears to make a necessary adjustment. (BTW, does anyone know any tricks for grasping and carefully rotating a cam w/o a special "Gear Holder" tool? Can this just be done with a wrench on the cam timing gear mounting bolt?)
Still, all this said, I DID confirm proper timing belt / gear positions before buttoning up the front of the engine.
And, also, from a practical perspective, last thing I want to do about now is spend another late night working in a cold garage... while trying to recover from the flu.