I have 95 mustang 5.0 v8?

i have 95 mustang 5.0 v8... i bought the car with a lot of performance bolt-ons already installed i have a vibration from 3500 rpm to about 6000 rpm and i dont know what it is...i changed the flywheel with an aluminum racing ram flywheel and the vibration seemed to have dimmed down some but came back about a week after the flywheel was replaced. i had a few people tell me it could be my harmonic balancer or dampner and i took a look at it and that thing wont budge or anything and has no play in it even when the car is running, the vibration does not occur when in neutral, anyone have any idea on what this vibration could be?