My 2001 Ford Excursion Limited 6.8L V10 4x4 has recently started a new behavior—after we took it into pep-boys for a brake diagnosis (why is it our cars always come out worse at these places than they go in?)

PROBLEM: At first the car would shudder as we drove up hills around 40MPH.

Fix attempt #1) I tried getting some Heet (Something like this on Amazon: HEET 28202 Iso-HEET Premium Fuel-line Antifreeze Water Remover and Injector Cleaner - 12 Fl oz.)

Outcome #1) This SEEMED to make it a little better, then it seemed to make it slightly better so I figured I should double down on that:

Fix attempt #2) I tried seafoam (Looked like this but I got it at O'reilly I just dumped it in our fuel tank

Outcome #2) Now the new symptom is that my car smells like burning matches when I stop the car. The smell seems to have diminished over time. The old symptom, the shudder seems to be worse now, I get it whenever I go about 35 - 45 on a straightaway. when I go faster, or floor it at that speed the shudder goes away.

Note:
Later on youtube, I saw people putting 1/3 in vaccum line on master cylinder 1/3 in gas line and 1/3 in motor oil—so I got another can of seafoam at the ready.

Here is what I have tested so far:
Vacuum Line test with Vacuum Gage: (my altitude is 4775 ft, so I am adding 4.7 in Hg to my measurement) I measured 13.3 In Hg at Idle + 4.7 = 18 in Hg. My book says at sea level good is 17-22 in HG so I think I might have a few minor leaks, but overall my vacuum should not impair my engine performance.

Any suggestions on what to look at next?
Thanks,
Jim