A couple of weeks ago we installed a new #2 cylinder coil pack on our new (to us) Navigator, and the 300hp, three-valve, 5.4L V-8 smoothed out like nobody's business.  But this had a downside. 

The problem was that we could then clearly hear (and feel) a persistent and extremely annoying thump in the rear suspension any time the vehicle was moving.  The Navigator uses an air-spring system in the rear with shocks and upper and lower control arms.  With the high mileage this rig has we knew that it was a toss up as to what parts were worn, though we were hoping it was something easy like shock absorber bushings (the shocks are the original OE units!).

We navigated to Attitude Performance in Arlington Heights, Illinois, where owner Matt Dinelli found that the two lower shock mount bolts had simply worn over the 10-year life of the vehicle, thus the shocks could freely move on the mounting bolts.  Surprisingly, the shock bushings and sleeves were fine.  Dinelli had replacement bolts in his Magic Parts Collection and was able to fix the problem on the spot.  Now the Nav is as quiet as a vault rolling down the road.  If only all problems were that easy to fix.