98 Expy 4wd issues

[Monkeylips]

I have a 1998 Ford Expedition Xlt. In 2wd it does fine. However, if I switch it to AWD 4hi or 4lo and then drive forward or reverse, not turning, I hear and feel this awful noise coming from the front end. I crawled up under it and noticed that the front differential is wet. Could low gear oil cause the noise. It's like a clunking, binding noise. Just trying to get some suggestions before I start tearing into it. Thanks

 

[Bowesmobile]

Yes, check your front diff fluid. Sounds like you have a bad input shaft seal. Replace the seal, top it off and try again. Also inspect the CV joints from the front diff for play by grabbing the shaft portion and trying to move it around. If it moves noticeably in and direction then replace the shaft.