Front end gear grinding noise

[jaciii]

This is something that popped up yesterday taking off from a stoplight. I had used the 4wd system a few miles back to cross a median (smooth gravel where the cops go) and was in 2wd again. As I accelerated I heard a gear grinding noise like to gears trying to mesh in the front of the truck. It went away after I got up to about 25mph. I haven't found anything through a visual inspection under the truck. This went away after a few miles, but recurs whenever 4wd has been used.

 

It's a 1995 k1500 Suburban with the shifter on the floor and 186,000 miles.

 

Any help/tips/advice is appreciated!