Hi and thanks for using 2carpros. Com.
The first thing I need you to do is check the front differential to make sure it isn't out of gear lube.
Where the front axles attach is the differential and not the transmission. I attached two pictures for you to see. The differential is internally lubricated with gear lube which is a very heavy oil. Since you have a leak, one of two things is happening. Either you are out of oil, which will damage internal bearings, or you will get really lucky and only need to replace a seal to stop the leak and then refill the differential.
As you will see on the attached pictures, there are several bearings in the differential as well as the extension housing, where the seal and axle come together. Any of them can be bad as a result of low fluid. However, I would first question the extension bearing closest to the seal that is leaking. It may have caused the seal to fail.
To check the fluid, there is a plug on the front cover of the differential. Remove the plug to check fluid level. The fluid level should be level with the bottom of the opening where you removed the plug. If it is low, refill it until it starts to leak from the inspection plug opening.
Here is the type of fluid the front differential requires:
Motorcraft SAE 75W-90 Premium 4x4 Front Axle Lubricant XY-75W90-TQL
Note: As long as you get the 75w90 premium type, it doesn't have to be the Motorcraft brand. However, make sure it meets the specifications listed.
Feel free to let me know if you have other questions.
Images (Click to enlarge)
Thursday, December 13th, 2018 AT 8:17 PM