Need help with this terrible vibration..transfer case? Rear?

[Xssv]

The truck is a 99 ranger 4x4 flex fuel 3.0 off-road.

Here is what it does, starting at about 45 you feel the vibration and it gets worse by 55 and even worse at higher speeds. It's not in the steering wheel, but in your butt and through the whole truck.

I put new brakes, shocks and it already had new front cvs and the other stuff done. I can get it going 65 and while its vibrating like hell, put it in neutral and it's still there. I did recently change the gaskets on both sides of the tail shaft extension and had the transfer case out to do this, but changed its fluid and filled it to the fill plug.

I just got the vehicle and have no knowledge of the previous work other than the front stuff is clearly new. Maybe they were trying to fix the vibration and quit.

There is some play on the rear end when you manually turn the driveshaft, about a half inch of rotational slack. No front to back slack. I joints all look good and feel good.

Here's why I think and hope it's the transfer case. Putting it in 4x4 causes the vibration to be really bad even under 40 mph. Turn the switch off and the grinding type extreme vibration goes away and then the main vibration comes,back with full force when you accelerate.

Best way to explain it is that it will even cause the cement to vibrate when you jack up the rearend and run it up to 70 with the wheels spinning in the air.

Could it be anything like the rearend or driveshafts? I'd hate to replace the transfer and waste the money if it might be something else.

Thanks to anyone who read/listened to this long post and any ideas are appreciated.

 

[RangerX]

Welcome to the forum!

When you did the tail shaft extension gaskets, did that change the problem at all, better or worse?
Is this truck a supercab, or regular cab?
You checked driveshaft play, in and out and rotational, how about up and down? Especially at the tcase, try pushing the driveshaft up and down and check for play that way.

 

[Xssv]

It's the super cab. The tail shaft had no up or down play. Was solid. I only changed the gaskets because it developed a bad leak after a long drive. The only play was rotational in the driveshaft. Had slack on rotation (about 1/2"). No up or down or in and out at the pinion.

 

[jmdirk]

the backlash in the rotation is fairly normal. I would say u-joints. But you said you checked those already.

Is the vibration worse under load/acceleration? And a bit better while coasting?

I recently had a similar problem. I had lost one of the retaining clips for a U-joint cap and the the cap was starting to walk out.

U-joints are cheaper and easier than a transfer case. You could start there. You do not want to lose a U-joint on the highway. I had the rear one right at the rear axle fail a few years ago. The long part of the driveshaft fell out. the short piece at the transfer case flopped around. In the time it took me to pull over the extension housing of the transmission had broke into several pieces