370ztime

Well I had my car back at dealership this week. Its an 09 370z touring with 6sp manual. Had my wheels road forced and new driveshaft but still at 70 mph on slight throttle i get this helicopter vibration noise in hatch area coming from rearend area. if i give it gas or let off all the way it clears up. Also at 80mph and above my car hums like crazy in the rear with 2 different sets of wheels and tires if you throw it in neutral at 100mph and let it coast down to 55 it shakes in back from 70-55mph then goes away..... I test drove 2 zcars identical models and they do not do it. Any ideas.....i dont just wana throw parts at the car.....some of you guys are super smart with the zcars so give me ideas please. thank you in advance.