I'm a driveshaft engineer, and after reading through this, my suggestion would be to send your driveshaft to a reputable shop that can check your ujoint clearances, shaft runouts (out-of-roundness), and imbalance at near highway rpm. The force generated by an imbalance increases with the square of the speed, and 400 rpm won't generate squat for an imbalance signal, so it becomes hard to detect the proper amount and angle to locate the correction weight. At 3500 rpm, you've multiplied that force almost 77 times. Now it becomes easy to locate and correct. If you want a recommendation, Dynotech Engineering Services in Troy, Michigan offers this service. These guys build shafts for the OE's and all kinds of motorsports. I don't know what they will charge, but they provide a great value and EXCELLENT work. When you say that it gets worse as your speed increases, this is tyipcal of a balance-related issue. If it was in your trans, you would most probably feel it phase in and out - that is, get worse then better then worse as you maintain speed. This phasing is due to different things spinning at different speeds within the trans. Hope that helps, good luck!