this is most likely TSB 06-4-4, here is a link
http://www.ford-trucks.com/tsb/fulltext/show_article.php?tsb=06-4-4
ISSUE:
Some 2003-2004 Expedition, 2003 Navigator, 2004 F-150 vehicles equipped with a 9.75" or 8.8" limited slip rear axle, and 2002-2004 Explorer 4dr, Mountaineer, and Mustang vehicles equipped with 8.8" limited slip rear axle, may exhibit a chatter, shudder, a binding sensation, or a vibration during low speed turns. On 4X4 vehicles, the sensation may appear to be originating from the front axle or the steering gear.
ACTION:
First verify the condition. On 4X4 trucks ensure that the vehicle is in 2WD, to isolate the rear axle. On all vehicles, turn the wheel fully right (or fully left) and then accelerate the vehicle. If the condition is verified to be in the rear axle, replace the limited slip clutch pack.
SERVICE PROCEDURE
CAUTION: INSTRUCT THE CUSTOMER NOT TO TOW WITH THE VEHICLE FOR 500 MILES (805 KM) AFTER THIS REPAIR, TO ALLOW THE AXLE TO BREAK-IN PROPERLY.
Order and install the appropriate clutch pack, follow the instructions detailed in the Workshop Manual.
6L1Z-4947-A (9.75" limited slip)
F5AZ-4947-BA (8.8" limited slip)
NOTE: INSTALLING A CLUTCH PACK INTO AN INDEPENDENT REAR SUSPENSION (IRS) AXLE IS DIFFERENT FROM INSTALLING A CLUTCH PACK INTO A SOLID-BEAM AXLE. BE SURE TO FOLLOW THE APPROPRIATE WORKSHOP MANUAL PROCEDURE.