Ok, I'm stumped. I've had this problem for a long time now and I'm looking for suggetions. (again) It started a couple years ago with a mild humming noise to a roaring noise and has got progressivly worse to the point where I'm ready to push it off a cliff. I replaced both left and right hubs, both cv axles, rebuilt the entire front axle with ALL new bearings and seals, upper and lower ball joints and front driveshaft u-joints. Checked to make sure the the front axle engagement / disengagement was working correctly and it is. Now I'm wondering if it's in the transfer case. My thinking is that because the axle is mounted to the passengers side of the frame, it may be transmitting the noise, vibration to the front end. Any thoughts??

 

The plunger on the transfer case vacuum switch might not be returning completely when you shift back to 2WD, or it might be stuck allowing a slight amount of vacuum to be applied to the front axle actuator instead of venting it to atmosphere, both are quite common. If the axle is partially engaged it'll hum, buzz, growl and grind like hell.

Driver side of transfer case, next to the drive shaft yoke. You can reach over the crossmember very easily to remove and check it. The spring loaded plunger needs to move freely.

It pays to shop around, here is the identical part at 3 different stores:

Dorman OE Solutions 600-500 - Vacuum Control Switch | O'Reilly Auto Parts

Buy Dorman - OE Solutions Motor 600-500 at Advance Auto Parts____

Motormite/Transfer Case Switch (600-500) | AutoZone.com_

 

I have a similar problem, same model year.
In my case the noise sounds like a loud grinding from the front right end, the volume and speed of the grind matches the road speed (faster speed = louder noise and click or grind), and when I engage the 4WD, it stops. Disengaging the 4WD sometimes stops the noise, sometimes doesn't. It also stops once I come to a complete stop, you can almost hear one final "grind" or click, and the everything seems to pop back into place. It will only do this first drive of the day - once it has done it it usually takes 24 hours or so for it to reoccur.
Any thoughts?
Same as the vacuum issue?
Thanks,

 

Same thing.... vacuum switch is not sealing tightly. Sounds like yours is starting to go. Better replace it before it destroys the teeth on the axle shafts and/or starts sucking fluid into the HVAC lines. 15 minute, cheap fix.

 

If this is the same problem on my 1997 2dr (and it sounds like it is) I will be EXTREMELY grateful I came here and did research rather than taking it straight to a dealership. Thought about buying a new T.C. off Craig's list, but I think the $15 15min idea better! Also, thanks for the links to the parts so we could all shop around!

 

No problem, glad we could help, let us know how it goes.

 

Ok, dumb question. How do I go about replacing it? I'm getting the part this afternoon, and likely will do the work this weekend, but I don't know the process for removal of, and replacement of the part? Any pointers?

 

The switch is located on the driver side of the transfer case, inboard and slightly above the front drive shaft yoke. From under the vehicle, looking front to rear, you can (barely) see the switch with the vacuum connector on the top of it. Reaching over the frame crossmember, from rear to front, you can easily access it. Unplug the vacuum connector and, IIRC, it takes a 3/8" drive, 15/16" deep socket and a ratchet. Loosen it up and unscrew the switch. Install the new switch and tighten it by hand, then finish tightening it using the ratchet & socket, install the vacuum connector. Now would be a good time to check the fluid level in the transfer case.

The image below is a manual shift transfer case, but you can see the connector for the vacuum lines just above the end of the shifter:

 

Just bought a '98 (my first) and I LOVE THIS FORUM! thank you thank thank you
I'm heading to the shop right now!

 

Thank you Captain Hook, solved a problem that's been annoying me for weeks, although the part in Canada is twice the US price, but at 40$, still a bargain.