1990 Ford F-150 — inside shoulder worn out while outside looks fine

If it were in my driveway, loaded-suspension checks on the 1990 Ford F-150 catch play that a wheels-hanging inspection misses. If the cheap test changes it, stop and reassess. Verbal 'we'll take care of it' is not a plan. I'd rather be boring and correct. That's my plan before I call anyone.

I wouldn't throw parts yet. Move a coil or plug on the 1990 Ford F-150 if it's cylinder-specific and see whether the code follows. That's the branch I'd chase this week. Had a similar mess a couple years ago and that's what finally sorted it. This is the version I'd tell a friend.