Well, joke's on me. Pennyman, you're exactly right.

I kept thinking there must be something I was missing and there was. Yes, I'd added Seafoam to the crankcase to clear up that sticky lifter, and the rest went into the tank which was right around the dose they say to use. It never crossed my mind that it would make the exhaust stink so bad. I know the spray can type does that, but never thought using it in the fuel would. Refilled the tank with fresh fuel and magically the stink is gone. Fine by me, I'd rather feel like a bit of a dummy than have to replace a head gasket.

The pros - Using it in the crankcase cleaned up the lifter tick pretty well. Using it in the fuel made a noticeable difference in how smoothly it runs and especially idles.

The cons - It stinks so bad I thought there was something seriously wrong. Makes a surprising amount of smoke when used as a fuel additive.

Thanks to all who contributed here, just thought I'd let you know the thrilling conclusion that the truck is just fine and I learned a lesson.