In past 4x4's, I've had good luck with valve cleaning to combat lack of responsiveness. I'm thinking of trying seafoam thru the intake. (Before now, I used to have tune-up shops de-carbon the valves.) This was usually good for 1-2mpg and snappier exceleration. (Part-throttle anyway)
Injector cleaning (or replacement) is another possibility. I actually have a set of cleaned injectors that I tested. I also have a set from a 40k unit that I'm running for comparison. Seem pretty equal -- and about 1mpg better than before.
I should probably sell one of those sets in the near future.
I think 100k is about the most you should push an OEM set of plugs. They'll still work but I wouldn't count on 100% performance.
2001 Ebony VX and 1989 Custom 383 Corvette