Vibration at higher speed, when not accelerating

[1985olds]

So I made half of this years Hot Rod Power Tour and overall was a great trip, met some great people, watched the late night hotel parking lot tattooing sessions, etc.

One thing that is bothering me though is that on a couple spirited runs on the connecting interstate detours I experienced a substantial vibration/hum that would occur when getting out of the throttle at higher speed (75+)

At 55/60mph everything feels nice and smooth (90-95% of the 900miles I rolled in 4 days), if I drop the fun pedal down everything is good and pulls hard (and with that low end gearing behind the overdrive like final gear in the 700R4 it certainly wants to find triple digits) but when I let out of the throttle I get a substantial vibration (rear view mirror shaking) and low frequency "hum" ... get back in the throttle heavy and it goes away, get the speed back down to ~60mph and it goes away.

My ride is a '69 camaro with Detroit Speed front end, art morrison "weld in" 4 link on GM 10 bolt rear with 3.42 gears, pulled down the road by Ramjet 350 (350hp) and 700R4 on 4 new HRBB wheels (19" in the rear, 18's up front) wrapped in Michelin Pilot Super Sports.

To troubleshoot this I assume I'm best to confirm balance on all four wheels, have pinion angle re-checked, but if those check out then am I looking next at torque converter, tranny, u-joints, something else in the rear end?

Let me know your thoughts and in what order (or if the conditions described above suggest something specific to you)

Thanks in advance!

Did you ever figure out your vibration, in going through pretty much the same thing? Been fighting it awhile.