Popping out of 4wd

[StarfishFarms]

This started as an issue similar to another post I see on here, but still a mystery. My mechanic of 10 years has tried several things, but still not sure what this is. I don't really understand the details, but will describe best I can. I've had this truck for a year. I use 4WD a lot where I live. It is a '99, has 125K on it. It is a 1500, 5.2L. This started as the 4WD light going on and off, when in 4WD. Now it slams in and out of 4WD, both in high & low. My mechanic checked all vac hoses, installed new actuator on transfer case(?). Today, because we're getting 14" snow, he slid the slider thing open for 4WD to stay engaged, at least for the storm, and put hose clamps to keep it there. No change. Someone suggested to him that the teeth (on something) may be wearing, and the fork is arching and sliding anyway? I might be missing something else he's tried. Anyone? I really miss my 4WD.