Going through this now on my own first gen bird.
As the front of the car specifically comes down, you end up changing the working angles of the transmission slip yoke to the driveshaft. Depending on the tire height in the rear, it's possible to have the transmission output, below the driveshaft pinion. If the engine is down the customery 2-3* you now have opposing angle from the transmission output to the driveshaft, which creates working angles beyond the recommended 2-3*.
You may want to try shimming the transmission upwards to reduce that working angle. All that said however, if the vibration is coming on at 60ish mph, there's a decent likelihood something else is going on. driveline angle related issues aren't typical at higher speeds. It very well could be a balance issue if it's higher frequency.
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-Jason
1969 Pontiac Firebird
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