Does my transfer case have a chain? (1999 Eddie Bauer)

[joethefordguy]

FWIW, from Wikipedia:

"
First generation (1997–2002) First generation (UN93)
...
Transfer case (4x4 models)
2-speed BorgWarner 4406
High range ratio: 1.00:1
Low range ratio: 2.64:1
... Expedition was available with automatic full-time ControlTrac four-wheel drive with two-speed dual range BorgWarner 4406 transfer case and a center multi-disc differential. ControlTrac was introduced in 1995 and included four selectable modes: Two High, Auto, Four High (lock) and Four Low (lock). Two High mode and Auto mode with auto-lock was not available."

as you can see, on this and many other sites on the internet (https://schematron.org/borg-warner-4406-transfer-case-diagram.html), your transfer case is a chain drive.

 

[Frank Swygert]

If you removed the front driveshaft and the issue stopped, it might be the shaft. Inspect the u-joints, or just replace them (they aren't expensive). Do you have the ControlTrac or standard 4x4? The sound is more likely in the controls, or if it uses an axle interlock with a servo motor on the axle to lock 4WD in it may be that. The vacuum servo on those can get weak or have a leak in the diaphragm or tubing (check the tubing first!) and grind when trying to lock in. If it has vacuum actuation I'd go ahead and just replace ALL the vacuum lines associated with 4WD first. The lines are old, and may just not be sealing well on the nipples any more, or have cracks if they use the hard plastic line instead of rubber tubing (most 90s and up vehicles do).