Front End Chirping

[Cheromaniac]

My Jeep suddenly developed an annoying chirping sound coming from the front end about a week ago. It literally sounds like the Jeep has adopted a pet bird, and the chirping increases in frequency with the road speed. Forwards or reverse it's the same. It's also the same if I coast. Only when I come to a complete stop does it disappear altogether. The sound doesn't change when I go round left or right corners and it isn't temperature related. The Jeep otherwise drives, steers, and brakes normally with no vibrations. If I jack the front right wheel up and spin it manually with the front left wheel on the ground, the chirping starts. The same goes vice versa. If it was the front wheel bearings, it would be strange for both sides to start chirping at the same time so I don't think that's the problem.
I've topped up the fluid in the front differential (it was about an inch low), greased the slip joint on the front driveshaft, and greased the front driveshaft UJ but to no avail. There's no play in the front driveshaft and no play in either of the front wheels (both 9-3 & 12-6 position).
I'm at a loss as to what's causing the chirp (please don't say it's the bird ;)) so I'd appreciate some ideas so I can further troubleshoot it.

[Mudderoy]

My Jeep suddenly developed an annoying chirping sound coming from the front end about a week ago. It literally sounds like the Jeep has adopted a pet bird, and the chirping increases in frequency with the road speed. Forwards or reverse it's the same. It's also the same if I coast. Only when I come to a complete stop does it disappear altogether. The sound doesn't change when I go round left or right corners and it isn't temperature related. The Jeep otherwise drives, steers, and brakes normally with no vibrations. If I jack the front right wheel up and spin it manually with the front left wheel on the ground, the chirping starts. The same goes vice versa. If it was the front wheel bearings, it would be strange for both sides to start chirping at the same time so I don't think that's the problem.
I've topped up the fluid in the front differential (it was about an inch low), greased the slip joint on the front driveshaft, and greased the front driveshaft UJ but to no avail. There's no play in the front driveshaft and no play in either of the front wheels (both 9-3 & 12-6 position).
I'm at a loss as to what's causing the chirp (please don't say it's the bird ;)) so I'd appreciate some ideas so I can further troubleshoot it.

I was going to guess axle u-joints, but I'd expect that sound to change in turns. Wheel bearings seems to be the only constant. So with one wheel in the air you couldn't pin point the chirp? Vac hose to the ear trick?

[Cheromaniac]

I just checked again a few minutes ago. I used my stethoscope and pinpointed the chirp to the front driveshaft. I'm almost certain that the noise is being caused by dried up needle bearings inside the double cardan joint at the rear end of it. Unfortunately that one's sealed and doesn't have a grease zerk so I'll need to have those two cross joints replaced (they're the original '92 factory units). At least it's an easy fix. :D