1st if it has that many miles its probably highway which isnt that bad. Is it automatic? The vibration is probably the driveshaft. Get under it and twist the shaft w/ parking brake on and see if the u-joints are bad. Dont go buying stuff trying to figure it out 'cause that can get expensive. Did you buy from a dealer or person. If from a person check to see if gears were installed. Sometimes after a gear swap the stock d.shaft will vibrate. Good luck brother and welcome to the world of stang bangers.
John
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89 Notch: ASP Pullies,
K&N Air,Transgo shiftkit,Trans. cooler,FMS 3 core , FMS 4:10's, 1.5" cowl, cold air kit, hooker shorties(jet hot coated), O/R H-Pipe, ASP solid mounts, Energy Sus. Trans. Mnt. BBK 70mm TB, MSD 6al, Dual elec. fans, 5.0 Firepower Kit, Scorpion 1.6 RR's, Flowmasters, 73 mm MAF, cobra intake(ported), 1" phenolic spacer,ported stock heads and crower #512 cam.