Put the scanner on it to see if there are any codes.  Major auto parts stores will scan it for free.  Hopefully, it's something simple like a dirty air cleaner, dirty MAF sensor, etc.  When was your fuel filter last changed?  After those, I'd be looking at those 16 year old injectors.

You’re likely getting misfires. I’m assuming It feels like a “fish bite” around 45 mph when you’re barely on the accelerator. If this is the case   You’ve likely got a misfire preventing it from going into overdrive. If you don’t have a check engine light illuminated It won’t show up in a regular scan (unless the scanner can show “pending codes”) but if you can access the mode 6 data you’ll see it 

 I had this issue for about a year and knew what it was but it was always on the back burner for me. Fast forward to last week and I got the fish bite hesitating (fluttering) right after I left for work but this time a flashing check engine light.  Turned around grabbed the other truck. Got home from work and scanned it with Forscan checking the mode 6 and found cylinder 8 with a bunch of misfire in mode 6 (still no check engine light or code tripped). Had I continued this time it was so bad it would have reached the threshold to set the light.
  You’re at 105k miles, they are “100,000 Mile” plugs. They’re probably the original plugs. If you do them yourself get the lisle plug extracting tool before hand and research the problems extracting those plugs from a ford engine. It’s difficult but it can be done. I’ve done this to now 4 5.4 engines and have had about 5 total plugs break in me. Extraction tool had them all out in under 5 minutes each time. 
  So far on this occurrence I only replaced the Cyl 8 plug will do the rest next weekend,  but the hesitation is completely gone.