So hang on, a sec. you're confusing the issue.Or at least you are confusing me,lol
>At one time you had a 904/3.91 combo and no issue. Now you have a A833od/ and same 3.91s and same driveshaft, and now you do have a vibration, at same road speed as the 904/3.91?
>And you corrected the speedO error with a different gear,right?.
>Further more when you push the clutch in and let the engine return to idle, at the troublesome roadspeed, the vibration persists.And the tires are the same today as they were with the 904/3.91s?Have I got that right?
Ok, then.
The 3.21s and the o/d trans combo may have masked the issue, cuz of the low driveshaft speed with the 3.21s. Had you buzzed the car up to (3.91/3.21x64=) 78 mph, the vibration might have been there, cuz now the overdrive is spinning 3.21s at the same rate at 78mph, as the 3.91s are at 64mph.
I got one test for ya;
>Is the vibration there in direct drive, at the troublesome speed?
If the vibration goes away, the issue is in the o/d system. How can I say that? Cuz the 3.91s didn't have an issue earlier, nor is the issue in the engine., nor the clutch, nor the tranny mainshaft, nor the driveshaft, nor the wheels and tires or axles ; cuz all that stuff was OK when the A-904 was in there,RIGHT? Well that just leaves the O/D , now doesn't it?
But if the issue persists, reset your axle end-play adjusters to near-Zero end play, and make sure they are fully seated.Then roadtest again.
If the ride height was changed, then the issue could be in the U-joint phasing being off, due to the changed pinion angle.
If the ride height was not changed, and nothing else was changed during the swap, and the issue persists, I would replace the rear tranny mount.
If the issue persists I would get after the pinion angle.
Here is a shortcut;
Caution, any time the rear tires are spinning make sure no one is back there ready to catch a 70mph rock in the face!
>Get the car up on jackstands;all 4 corners. The rears under the axle tubes,and the car leveled out to the same rake as it was on the ground. Buzz it up to the troublesome roadspeed,64 mph. Is the issue still there in both O/D and in direct? This is a verification test.
>If the problem is gone, get some different tires and retest, wheels on the ground this time,as in roadtest.Problem gone means those removed wheels or tires are the source. See below:
>If the issue persists,note the engine rpms in both those gears. Clutch it and brake the wheels to a stop.Put it in neutral,and Buzz the engine up to those noted rpms, with your foot off the clutch pedal. No vibration: bring the engine back to idle.The problem is not in anything from the clutch forward. This is a verification test.
>Again buzz it up, this time to70mph, and pop the tranny into neutral and shut off the engine. As the roadspeed comes down through 64 mph, note the vibration level. If it's still there in neutral, the problem is not in the tranny.So now we have proved it is from the driveshaft to the wheel/tire assemblies, or the tranny-mount is no able to suppress it.
>The wheel/tire assemblies are the most likely, so take them off and secure the drums. Again buzz it up to 64mph. Problem gone?,problem is in those assemblies.
>If problem still there, remove axles, and retest.
>Problem still there; Well it's either the driveshaft or the chunk, or the pinion angle.The driveshaft you already covered. You said 3.21s so I'm assuming you don't have an 8.75, so swapping the chunk is out. So that just leaves the pinion angle, and the tranny mount. Tranny mount would be my first bet.
Now;as to those wheels
Check for tire hop, and also the wheel runouts, and the center register.Tire-hop cannot be corrected for in the balance.Out of round modern tires is usually a tire going bad inside, and it's best to get rid of it. I have driven on very old radials,that were still round, so don't buy the "your tires are too old" line. However, tire hop may be the first sign that the tire is failing inside, and while a radial rarely fails catastrophically, I have experienced rather rapid loss of air.
>Another thing; your wheels aren't unilugs are they?,lol
>Oh and the pinion preload. Make sure the pinion has no side play to it;ZERO. Sometimes over time the pinion nut can work loose.
>And finally,the driveshaft runout has to be held to a very small tolerance. I don't recall what it is, but if all else fails......Your man did say, your shaft had some of that.
>If it makes you feel any better, I ran 4.30s for a couple of summers that had a similar issue; just faint enough to be noticed at 62mph. I never could completely get rid of it, and Boy-O-boy, I tried hard. It was completely gone with the current 3.55s, no sign to well past any speed I have ever had the notion to try, even with the windows rolled up tight!And the only thing I changed was the chunk.
>Ok so I hope I covered it all, and all correctly. Happy hunting!