I've changed the universal joints on all 3 ... now between 30 and 40 mph... I have this shuddering vibration ... what could it be ??? ... I've double checked my work, and all seems to be in order ... need to say I have a 97 XLT 3 door long wheel base

 

A carrier bearing that is loose in its rubber mount can cause it
Meaning the rubber gets loose in the steel u shaped mount
How is the transmission mount?
A broken trans mount will cause it
See if the driveline moves up and down where the carrier bearing is
Ford used to have us shim the carrier bearing to be tighter in the rubber mount
I fixed a bunch of them doing that shim procedure

 

thank you for a reply on what to look for... I have a question .... what is the carrier bearing??? ... and where to look... don't get me wrong I enjoy twisting wrenches ... just through the years names have changed... like the control arm use to be called A-Frames ... things like that ... I really thank you for the reply

 

Carrier bearing
The bearing in the middle of the driveline
Center support bearing is another term for it
Right about the nomenclature, now days they call some of the lower arm bushings Compliance Bushings
If you have 3 u joints and no dual cardan you should have a center bearing

 

I had a chance today to look for things you suggested ... and here is what I found ... there is a little up and down movement at the carrier bearing.. when i say little its little without using much force ... I look towards the trans ... the mounts look good ... however where the shaft enters the trans there's a lot of slack there up and down and side by side .... Is this normal??? ... all of this started when the universals where changed ... before that it rode smooth as silk

 

Also and since it occurred after the change, and IF you have the H9 set up (LS) in the rear end..consider the rear gear fluid may have broken down and needs servicing.

 

Ok... have to ask... how can i tell if I have the H9 set up ??? is there markings or tag to look for ???