11 GMC SIerra SLE 4WD

[stealth_weapons]

This is my wife's truck and I am having some issues that I am wondering if someone can help me out with.
My wife drove the truck to work (about 50 miles from home) the other day and the truck drove fine. On her way back it started surging and making a clunking noise that can be somewhat felt in the floorboard.
I drove out to where she pulled over and test drove it myself. I got the same results at speeds of 60mph and higher. There isn't any noise when turning at low speed. I did notice the 4WD selector was in auto. the weather was nice and the roads were dry. I ended up limping the truck home and playing around with the 4wd selector and I cant hear the truck engaging into 4wd but it is in fact engaging.
I checked the transfer case fluid and it was full and not burnt. the front diff however, was extremely low and the diff was covered in grease and grime. I pulled the drain plug and very little gear oil came out and the magnetic tip was covered in metal shavings. I cleaned the plug and refilled the diff.
the truck still made the noise when I took it out for another test drive. it is weird. its not the same noise as the frozen slip yoke I have read so much about. when you drive it at 60mph or faster there is a clunkish sound that doesn't have a pattern to it. it will make the noise for a bit ten stop then it may or may not start doing it again.
I cant figure out what is going on with this thing. any help would be appreciated. and the truck has 111K miles on it 5.3L flex fuel with the 6l whatever trans that has the manual shift option.