brittle

My 98 F150 makes a loud clunk after shifting out of 4WD, not immediately, sometimes not at all. It shifts into 2WD properly then a couple blocks later there's a loud clunk and it sounds like the transfer case gears are grinding. This has happened when going from icy, snow covered roads to dry pavement, shifting to 2WD just before the dry road. The 4WD light goes out and it feels like its in 2WD, but the clunk from the t-case sounds like the gears were still engaged. I'm sure if this happened under heavy load something would break. T-case fluid level good, no binding in shift linkage. Anyone else have this problem?