Scott,
The vibration is caused due to the pinion angle on the the rear control arms. I purchased one of the last sets from SVPU, which were actually made by Kenny Brown. I actually purchased the whole suspension control arms from SVPU..what a world of difference.
I am not sure what you are referring too with the "beltline trim", but if it is weatherstripping on the upper portion of the doors, then yes, those the one of the infamous parts to ruin the exterior of the Mark VII.
I purchased a newly created set, from Batman1991 on Ebay. They go for $300 a pair..well worth it in my opinion. In NY they were $400 each at the dealer.
I know what you mean regarding the stop lights...with the 400+ rwhp I have in the BMNLSC, I love smoking those unsuspecting kids in their alleged hotrods...
BMN