Tune up or not?

[John Christopher]

I would not change the plugs unless it starts to miss. I've had an 03 for 7 years and had a coil go bad and had it replaced. Then another one went out a few months later so I had 8 new plugs and coils put on. It's difficult to change plugs on the 5.4 especially in the rear of the engine.

 

[Snag]

I agree with bedrock, at 100K its time for service and proper maintenance. Plus you know no history of the vehicle it sounds like. Its time for all new fluids in the differentials, transmission "and filter", transfer case and maybe engine.
Certainly time for a plug change or at least a pull and check and check to make sure each is tight before it blows out. And often you find the front easy ones have been replaced, but not the back ones. That belt runs everything, a broken belt means you are stopped or blowing an engine. They are simple to change, and check your bearings on all the devices the belt runs on, this could be your noise.