4th gear 2800--3K rpms

Well;
4th gear at 2800 is about 65 mph with 3.55s. Are you saying ;
65mph in third gear does not illicit the vibration?
Or how about 2800 in third gear at ~45mph;
how about if you just lightly back off the throttle, or lightly lay on the throttle?

If no vibration on the; third gear @2800 test; that sorta proves it is not related to anything forward of the slip-yoke. But to be sure, try; up to a couple hundred rpm either way from 2800.

If the vibration originated in the rear tires or rearend, it would continuously increase in both frequency and intensity, directly proportional to speed. Since you don't mention that, it sorta proves that the vibration originates somewhere else.

That doesn't leave a lot of other sources. But;
If you can feel it in the steering wheel;
it is usually found in the front tires, like a shifted or blown cord.
If the steering wheel develops a will of it it's own, that would be the alignment, usually in the toe, but could be other, and occasionally it is aggravated by worn parts, altho the worn parts are seldom the cause of it.
After that, I think there are only two other possibilities;
1) the various vibration isolators; like engine and trans mounts, and spring bushings. Again these are not the cause of it, but only allow a pre-existing condition to to be discovered.
2) Or the driveshaft;
If this just came out of nowhere, look for a bent or damaged, one.
If it has been developing over time, look for shifted cups, or a loose X-pin.
If it has always been there, look for U-joint phasing.

Well; I just thought of another; If you have recently installed two different rims on the front with different backspaces, this will cause the left and right scrub radius's to be different, and they will fight for dominance.
Same goes for matching wheels that are offset to the outside, or taller than factory wheels with a non-complimentary offset; which all destroy the factory scrub radius. If your tie-rods are loose in this situation, the front wheels will "shimmy". A loose steering box won't help any either.

Or another one I have experienced ; is a raggedy clutch disc,or sticking flyweights in a diaphragm type. The sticking flyweights is a PITA, cuz sometimes they don't and everything is fine. Then the next time I fly thru the gears, there it is again. I always notice it in 4th gear at hiway speed, because I just got to hiway speed by revving third to 4000, and dropping in at 2870 for me (@65) , so the flyweights were just recently out for a ride. I can usually get rid of it, by hitting neutral and gently blipping the throttle. Sometimes it takes several trys.
If it doesn't go away, for me, that usually spells a fragged disc, so I schedule a replacement.
I like factory 340 discs, and always have a spare in stock. They don't last long in the CenterForce PP, but as a streeter, I like the organic feel and I think the small penalty of replacing them every so often is acceptable; altho I no longer sidestep the clutch; Sidestepping, usually/eventually, busts/cracks the hub and she spits out springs; now I know it's time.
Well sometimes I do sidestep the clutch, but only to show off from ~zero mph, and very low rpm, to show the dump it and go personality of my 367combo.

Happy Hunting.


Some of these things, I see, have already been mentioned.