Vibration on accelerating above 45mph, especially in 4th.

[karbon]

I've done various searches and the only result that described my problem didn't have much of an answer.

The car vibrates when the speed is above 45MPH, and I start getting on the throttle to accelerate. The problem isn't present when giving just enough throttle to maintain speed. The vibration is especially violent in 4th gear for some reason, though 5th is pretty close. I only get a bit of vibration in 3rd gear above 45mph. If I let off the gas, the vibration stops. Likewise if I get back on it, it starts again, like a switch.

The thing that makes me not suspect any kind of tire or wheel balance issues is that the vibration is only present when in gear and accelerating. That, and it's interspersed with a random thumping. The vibration feels like it's side-to-side, but it's hard to tell for sure, and it feels like it's coming from the center of gravity; not the front or rear of the car.

I jacked the car up a couple days ago and checked to see if there was play in my carrier bearings. I could only wiggle the driveshaft about 1/4" back and forth if I really forced it. It just felt like I was fighting the rubber boots and the assembly wasn't "loose". It might be worth mentioning that 4th gear pops out if I accidentally don't push the shifter all the way into 4th position, usually while shifting too quickly without paying attention. I've never had a problem with any other gears though, and it might just be a coincidence.

 

[wazzelby]

Have you done a timing belt recently? I've felt this when someone put the timing belt on with the balance shafts 180 degrees out.
Also, have you checked the fluid in your diffs (front and rear)?

 

[karbon]

No on both counts. The timing belt has been on there awhile and it still in great condition. I did the rear diff fluid 2 summers ago and recently changed the transfer case fluid this past May when I did my steering rack replacement. I understand why wearing diff parts would cause vibration but not random thumping but I won't rule out anything because I'm not terribly experienced.

I considered the balance shaft thing too when it was just a vibration in 4th. It's since degenerated to happening in the top of 3rd and all through 5th accompanied by the random thumping. THat's what made me discount the balance shafts (I'm taking them out soon anyway).