The stock, tired 283 in my '62 Suburban hums along at 70mph very nicely. the previous owner told me he thought the dana 44 rear end had 4.11 gears.
There's a bad vibration that kicks in past that.
It's somewhere in the driveline, but not sure where yet. Could be u-joints, but I doubt it. motor mounts, or the engine is out of balance for whatever reason. Getting a new engine and trans this winter.
-Brad
