Pinion Angle/Tailshaft Bushing

Assuming no tire changes,and no ride height changes,with the new gears, only one thing changed; the power train speed,at any given mph. That is from crank to pinion gear.
If you previously had 3.23s you are now at about a 21% higher rpm for any given mph. ( the Speedo was corrected, yes?). So the vibration, if pre-existing, should have been there at a 21% higher speed, with the old 3.23s. This maths out to about 67 mph, previously, as compared to 55mph, now.
If you did not have an issue at that earlier mph, then I would have to ascribe the issue directly to the chunk, or that the new rpm is exciting the natural body oscillation.


Your thinking is in line with mine. Cruising at 65 mph with 3.21s was 2000 rpm and that is usually where I hang out on the highway. Now, 2000 rpm is 55 mph with the 3.91s. So, I need a bit more engine speed and driveshaft speed with the 3.91s to get to the same 65 mph, which might be why I am now noticing this sensation. The driveshaft is 3” and .083” thick, so half critical speed shouldn’t be anywhere close at 65 mph.


No tire changes were made. I thought about the rear having the issue after the gear swap, but I don’t feel much of the sensation/vibes coming from the mid driveshaft to rear area at all.


During lunch, I put the car in 3rd and ran it up to 60 mph. Not near as much noticed in that scenario, but that trans gear is 1:1, so the driveshaft speed is a bit slower.