I believe this is the thread I was thinking of, don't know if it applies in your situation.
https://www.ford-trucks.com/forums/3...tml#post209834

 

welp may have found the problem .... went to start taking the tail shaft off and noticed the crossmember frame bushings had a lot of movement in them the rubber is cracking.

 

Ok, good find & feedback.

 

Looks like the only way to fix it is to buy a whole new crossmember ....

 

My 99 4X2 4.0L SC, auto is a one piece aluminum drive shaft, so I don't know squat about the two piece set up but maybe a member with the 2 piece set up will chime in. I remember reading in the above thread about the OP saying he had replaced his carrier bearing but don't know if this applies to your set up.

 

I have replaced the carrier bearing twice once when I got the truck and again when I installed the limited slip carrier . Only used timkin/Torrington bearings .

 

Well boys and girls I made some progress finally !
No question the cross-member bushing are shot but wanted to make sure that was the cause of the vibration so I took some wood shims and put them between the frame and cross-member to take the slop out of the bushings and it worked the vibration is 90% gone as the cross-member can still move side to side.
https://drive.google.com/file/d/0B1v...ew?usp=sharing

 

Great feedback with pictures. Feedback is what makes these forums so valuable...........to someone in the future.

 

This crossmember is only used on the 2wd ,the 4wd bolts directly to the frame so check the rubber mount on directly on the transmission as well .
With all the new parts she'll be good to go for another 200k!

 

Way to hang in there on your trouble shoot to see it through to a confirmed ending. Let us know how the replacement parts do.

 

Hopefully to close this thread out , I finished the cross member last week has to source new bolts ( order new bolts if you'r doing this ) took her out or a quick spin and no shake on take off but didn't get up to highway speed due to finding out the lower intake gasket failed again and is leaking coolant that's this weekends project .
Hope this helps some one out.
thanks for all the help!

 

Ok good feedback & to hear it seems you've put the fix on the vibration problem. That bushing being bad & letting the tranny tail end move around was likely a good suspect for the two previous carrier bearing failures.
Bummer to hear you think it's developed a lower manifold gasket leak.

 

Well I spoke too soon, took her out Christmas morning after replacing the gaskets and she still shakes .
Tonight while fixing a leaky coolant hose I noticed the passenger side motor mount is sitting closer to the frame then the driver side.
Guess ill be starting that this weekend.
Is there a write up on doing the motor mounts?

 

Could be a warped ring and pinion! Had that problem in my new Mustang GT and they replaced them and the driveshaft and it cured the problem!

 

Ok so I ordered new motor mounts had them overnight delivery.
after looking at the old one and new ones couldn't find anything wrong , the passenger side has less rubber then the driver side.
put a jack under the engine and the mounts didn't separate from the frame.
the bolts that hold the motor mount to the block were backed out tightened them up and she rides smooth as glass at 60 mph .