Well, here's an update:

Before I made the trip home (360 miles, all freeway), I played with the idle & timing some, didn't really change it.

I pulled 21MPG on the trip home, so city driving was most of the problem, BUT...

I did take it to the dealer for lube/oil/filter and a general lookover, and here is what they found:

1 ruptured diaphragm in the vacuum advance (I asked them to check)
1 small vacuum leak between the intake manifold and the head in the vicinity of the #4 cyl (found when they set the timing and idle)
Leaking cat to exhaust flange, I knew about that, and had them throw a new gasket in

So I guess the moral of the story is a lot can be attributed to city driving, but if your mileage drops, it's still a good idea to look into it.

Now I'm wondering how much it will improve when I fix the diaphragm and vacuum leak... if I can pull 21MPG doing 75-80 on I5 with those problems and an exhaust leak, I can only wonder what it will do afterwards.

Thanks to everyone for all the helpful ideas and good suggestions. This board continues to be the greatest resource for shadetree mechanics I've ever found.

My Truck: 1987 XtraCab DLX 22R 4WD 5 Speed Manual
"Speed has never killed anyone. Suddenly becoming stationary, that's what gets you." -Jeremy Clarkson