My 88 is a 5 speed 5.0 liter and when I'm doing over 55 in od there is a constant thumping noise and it gets worse when I let off the gas until it gets back down to 55 then it drives smooth again any ideas on what to look at? Things been sitting for 17 years and now is my daily driver as of 2 weeks ago

 

Mine does exact same thing. If you figure it out please let me know. Been driving me crazy for months.

 

my 94 was doing that. turned out the rear U-Joint was shot. you might want to check to see if you have any excess play in your drive shaft. just an idea.