Stephan, just went through a "similar" situation. My tires balance good, looked good but the faster I went the worst it got. Short story is I just took off 4 tires with less than 3000 miles on them and put on a new set. Problem solved. The bad tires were new 5 years ago but have set with weight on them most of the time. Strange thing is we checked the balance coming straight off the vehicle and off only 1/2 ounce....but bounced/shook/rattled like crazy. I could have had them trued up but why waste good money and cut off all the rubber.
I'm not saying its your problem but sometimes it takes a good tire man to detect this situation. My next GUESS would be take your driveshaft to a driveline shop and have them check the joint and spin balance it.
Good luck. Let us know what you find.
