YES, and NO. Did you remember to change your speed-O gear? Is the 65mph that you reported a true groundspeed of 65mph, or an uncorrected speed-O reading?
If this is a true speed, then;
You have increased the driveshaft speed 22%. The vibration used to be at 53 Mph, but the intensity was much reduced. And you may not have noticed it, cuz who drives 53?
But to prove it, you will need to do some detective work.
Even up your tire pressures and drive the car around until everything is up to temp and the tires are round;probably 10 miles for the tires. Bring it home jack it up to get the rear wheels off the ground, put the jackstands under the axle tubes, pull the jack out.This is important; that the driveshaft to pinion angle be in the normal working relationship. Have a helper start it up and run it up slowly while you eyeball the tires for roundness. This only takes a few mph.Also check for a bent rim, and rim runout.Since you didn't mention a problem before the swap, and the wheel speed remains the same, I expect your wheels and tires to be fine, but this is where I start.If everything checks out:
Then run it up to 30mph.Do NOT stand behind the car anytime the wheels are in motion. If the rear end starts to bouncing,or you see the springs quivering, go get your wheels balanced first.
Now starts the test. With round tires and true rims and balanced wheels. Run it up to 30 again.Do NOT stand behind the car anytime the wheels are in motion.All good? Put it in Direct gear, and Continue until the vibration becomes apparent. Record the speed.Stop the test.
Remove the wheels. Secure the drums. Repeat the test. Record the speed, Stop the test.Any difference? If no; remove the drums. Repeat the test,record the speed, stop the test. Any difference?If no; Normally I would say to remove the axles and repeat the test, but with an 8.25 this is a PITA. Plus I only ever found one bent axle with this test.
So now we have eliminated everything up to the ring gear.And you,when you performed the coast down with the engine idling,eliminated everything from the cluster forward. So the vibration has to be from the tranny mainshaft to the crown gear.Now let's see; I've never seen a bent M/S and you had no issue before the gear swap, so lets eliminate that. Next up is the pinion angle, but that was also fine before, so now we are left with driveshaft, u-joints, and crown/pinion. C&Ps are precision manufactured items. If the gears are quiet and the backlash is not too tight, then your installer did a good job, and I would never suspect them. So next up is the U-joints, and the propeller. So put the tranny in neutral and with the engine off, rotate that hummer and see if it is bent.Check especially the backend to be sure your guy installed the rear straps correctly. If it spins true,remove that D/S and check the joints out. No don't take 'em apart,yet. If the joints operate smoothly, lets rule them out. But before we do, pop the rear caps off one-atta-time and make sure there are no rollers laying flat across the bottom. While they are off you can check the crosses to see them all shiny,smooth, and well-greased.
Ok that just leaves the balance of the shaft...... and the unchecked axles.
At this point it's time to put a dial indicator on the flanges and prove them true. And if they are,that will get us just the D/S to blame. So it's off to the machineshop.Bring your wallet.
You can leave the car on the jackstands for a couple of days where it is; right? Don't bother putting anything back together,cuz when the ds comes back, the first thing you are..... ,ok the second thing you are gonna do is repeat the test, to prove the issue is resolved.If yes, THEN you can reassemble everything and go raise a lil-El. If no the issue is still there, you are gonna have to revisit the pinion angle.
