This forum is GREAT!

We have same problem on a 99 F-250 SD SC 4X4. In addition we know that we have a vacuum link someplace.

Vibration that is felt and visible on steering wheel with hubs manually locked and SOF ON!

Tests:

1. Hubs manually locked and SOF OFF running on smooth highway all 20 to 60 MPH - NO Problem!

2. Hubs manually locked and SOF ON vibration in steering wheel starting around 20 MPH all the way up to 60 MPH.

3. Unlock hubs, NO Problem at any speed with SOF ON or OFF.

4. Had truck on 2-post lift and we have just enough vacuum to "lock" into 4X4 mode (hubs NOT manually locked) with front axle but not enough vacuum for hubs to lock and hold when on the ground (new snow over gravel driveway). Rear wheels slipping but fronts just rolled along, multiple tests with out turning 4X4 OFF.

5. While on lift, turned SOF OFF, put truck in reverse to drop 4X4, Hubs unlocked when trans put back into drive.

6. Checked all u-joints and Cardan joint for bad bearing, did not see or feel any problems, YET! (hand rotated drive shafts and looked for bearing play or binding).

From info on this thread it looks like we have a bad u-joint bearing someplace or the Cardan joint.

Truck is on way to college (130 miles) with 18 year old son. He will be back in 10 days after his finals.

Thanks for any additional help.

Ron - Former STM (Shade Tree Mechanic), Shade Tree blow down so built a 25 X 32 ft shop with a 10,000 lb two post lift.