Help when I drive my 1999 f150 starts to shake well more like a really hard and loud vibration around 70 and won't go any faster

What is your question?

When I bought the truck the guy that sold it to me said that the speedometer was off by like 15-20 mph because it had oversized monster tires on it. Not for sure if that's a thing. It

What makes this problem better or worse?

If I keep the pedal to the floor it gets worse

How long has your 1999 Ford F-150 had this problem?

Well probably the whole time I've never got it on the freeway until today I bought it 2 years ago