So I picked up a 98 limited with 146K on it. It does this strange thing where it vibrates (much like driveline vibrations) at about 3k rpm or so. It normally does this right before shifting gears, so at first i thought it was the trans, however i revved the engine up while in park and it does the same thing. It also idles a little rough. Possible causes? Fixes?

Thanks

Harmonic balancer maybe?