^i think your right about the driveshaft if they changed your pinion angle at all while installing the pinion gear and lsd gears. In highway all was fine until 65 mph then vibration gets worse and shakes car worse in relation to speed increase. At 80 it was downright unnerving. I have adjustable lca's and they were installed slightly off. As in both wheels were .5" further back from center In the wheel wells. I shortened up both lca's (which would have slightly changed the pinion angle) which recentered the wheels perfectly and all vibration and buzzing is gone. Maybe this is your problem too.
2012 Pony Pkg 3.7 convertible, MPT tuned, 4.10's, Dynotech 3.5" DS, Borla Touring axle backs, BBK 73mm TB and ceramic shorties, Whiteline sway bars and UCA/LCA with relocation brackets, BMR adj. PHB and brace, Steeda CAI, CE frame braces, Bilstein shocks, H&R race springs, SS brake lines, full HID, GT 500 gauge pod with gauges and misc. exterior upgrades