I had a leaky front differential and I fixed it over the weekend. It was my first time but I'll share what I have found from various sources.
2004 f150 stx
material
royal purple 75-140 weight (1.8L front). RTV red high temp gasket maker (I hear black works too). torque wrench (24b) from harbor freight. wire brush from harbor freight. pan and pump mechanism for gear oil from auto zone. brake cleaner.

steps
1. removed the fill drain plug (always do first)
2. removed the bolts off the front plate and let oil drain into pan.
3. remove front plate and cleaned it thoroughly with brake cleaner.
4. removed all gasket material with wire brush, put paper towel over gears to prevent gasket from getting in.
5. practiced putting the front plate onto the differential
6. put a thick bead of rtv around the seal and inside of the bolts not outside.
7. put the front plate on and tightened hand tight.. I bought the wrong torque wrench size.. needed a bigger head to get on my socket.. dope 24lb is spec
8. 1 hour later tightened the bolts just a few turns past hand tight until some rtf oozed.
9. 24 hours later I pumped 1.8L of royal purple into it (make sure vehicle is level).
10. No leaks yet. Good luck.