Found an 05 xt manual

So I came across an 05 with 99k miles :shrug: I'm a little worried about the miles. I took it for a test drive and rodded it pretty good, checked the oil and it looked clean. The big thing was it had a bad vibration at about 75+ made the rear hatch shake pretty bad. I couldn't feel it in the steering wheel, just through the seats. The tires looked good, but I've never had out of blanced tires shake that bad. Any ideas or things to check?

Thanks