How do you repair intermittent shifting between 2WD and AWD-4WD of a 1999 GMC Suburban 1500 SLT during wet weather?

If you have the electrical (thermal) front axle engagement setup, it losses it's abilty to engage the front axle over time, and really has problems in cold weather. Replace it with a mechanical setup from "4x4 Posi-Lok". Their stuff is pretty much bullet-proof.