The problem I'm having is a fairly strong fast vibration (coming through the seat, nothing at all in the steering wheel) starting at 55-60, getting the strongest at 70mph, and then slowly going away above that to 80.

Then comes back with a vengeance about 110mph or so.

No matter what I seem to do, it never seems to have any effect on the vibration. I have bought the best driveshaft that driveshaft specialists make, with best joints, upgraded the pinion yoke for the larger u joint, had different rims and tires, balanced the rims and tires at different locations with different machines and different tires. Roadforce balanced also tried.

I have a 2 inch drop spindle in front and 2" drop leafs with 2" shackles in rear.

I have done, no shim, 6 degree shim forward and backwards, 2 degree with fat end back. No matter the shim the vibration is always exactly the same severity and mph.

That leaves me thinking that it has to be something betweend the wheels in the axle housing but i'm not getting any noises from there, etc.

I'm just looking for any more suggestions at this point as to what it could possibly be?

Don't know if something wrong with the parking break drums could cause some sort of issue like this, somehow a warped rotor, dragging caliper/brake pad, bearings in axle/differential housing, crush sleeve?

After that the only thing I could come up with would be the transmission somehow...

I don't know and its driving me crazy because I don't want this vibration to destroy something like the differential, driveshaft, or transmission since I'm making decent power now

Alright, I've got a 1999 GMC sierra rcsb that originally came with a 5.3, yada yada yada.

Most importantly a built transmission 4l60e with vacuum modulated valve body, 3600 yank pro truck converter, Drive shaft specialists 5" aluminum high speed balanced driveshaft, with spicer xl u joints, rearend has 4.10 gears and eaton clutch style lsd with new conventional fluid and friction modifier.

Mods other than above:
Now it has an lq9 with ~10.5:1 compression, WCCH stage 2 heads, cam, ported nnbs intake, 92mm fast dbc tb, 100mm Ling. MAF, 4 inch intake tubing to custom cold air, pacesetter 1-3/4 headers, offroad y, full 3" exhaust, 3" cutout.


Anybody know anything? or anyone that can help?

 

Your lockup clutch could be goin bad and making a vibration in your converter. I've had this happin. Also check your output shaft int he trans for any up and down play. It should have a very lil bit only. Your bushing could be bad in the tail housing to. Last check your harmonic balancer and motor mounts.

 

It shouldnt be the lockup clutch because this is a brand new converter and it also had this vibration with the previous coverter.

I can check the output shaft play, but other than that the tail housing bushing is the black one that u see where u insert trans yoke from driveshaft?

Not exactly sure what to do for motor mounts other than just replace them...and harmonic balancer is an asp underdrive pulley...dont know if I should just buy a new one or something.

Any other suggestions on what he said or other things that could be a possibility for the vibration?

 

DS Center Support Bearing?

 

its a regular cab truck with a one piece driveshaft

Unless what im thinking of isnt the bearing that you are thinking of?

 

Bump for more suggestions plz