FWIIW, the Ford service manual does NOT call for a differential gasket. Clean both surfaces and use a bead of silcone around the cover. Torque to 32 foot pounds.
NOTE: In-vehicle repair refill capacities are determined by filling the rear axle with the specified lubricant to 6.4-14.3-mm (1/4-9/16-in) below the bottom of the fill hole.
>> Installation
CAUTION: Make sure the machined surfaces on both the differential housing and the differential housing cover are clean and free of oil before applying the new silicone sealant. To prevent contamination, cover the inside of the rear axle (4001) prior to cleaning the machined surface.
Clean the differential housing and the differential housing cover gasket mating surfaces.
CAUTION: Install the differential housing cover within 15 minutes of applying the silicone, or it will be necessary to remove and reapply new sealant.
Apply a continuous bead of sealant of the specified thickness to the differential housing cover.
Use Clear Silicone Rubber F7AZ-19554-CA or equivalent meeting Ford specifications ESB-M4G92-A.
NOTE: If possible, allow one hour before filling the axle with lubricant to make sure the silicone sealant has cured.