Unfortunately these things are known for leaking. I would suggest to remove the front differential and replace both seals and reseal the case. The last time I had my engine out I replaced the outer seal because I noticed it was dry rotted then after about 2000 miles I started getting a leak that looked like it was coming from the oil pan but was the inner seal leaking. These are a royal pain to remove from the truck, so when you have it out learn from my mistake and do both seals and the case. The case is very easy once it's out of the truck. Lay it on its side and remove the bolts and the case separates. Both of the ones I did didn't seem to have any kind of sealant at all on them, so I used a very thin layer of black rtv on it and bolted it back together. It's been about a year now and no leaks. Good luck
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