Might be getting somewhere.... the double cardan type U-joints use a
centering ball between the two cross and cap joints... these can develop
wear that can make all manner of noise with no real obvious vibration.

With the shaft off the car, move the joint through it's full range of
motion.... there should be a slight resistance build as the joint approaches
the straight position and the joint should have smooth action. With the
shaft installed in the vehicle, grasp the shaft next to the U-joint ass'y
and move it up and down (can take quite a bit of force) - there should be no
play at all.

If this joint has play, you will probably need to look to the aftermarket
suppliers for parts since Ford doesn't appear to stock or supply any
centering ball kits. Plan on replacing the associated U-joints at the same
time. When reassembling the joint, be very careful not to "cross up" a
bearing cap needle - I've repaired manyu of these style joints over the
years and there is still the odd one that tries to be as frustrating as it
possibly can. If your local auto parts store has a C clamp type ball joint
press for loan or rent, the C clamp can be used alone to aid in
removing/installing the U-joints.

HTH