HELP!!!!Put in 8 3/4 rear and it vibrates real bad

Sounds like you have a 5380 series end yoke, which that center would have been from a truck.
7290 is not 4" even across the outside.

A crooked wheel won't make that vibration, but sticking a 7260 or 7290 where a 5380 should be and you'll lose your teeth.

.