Servicing 4x4

I have a 2004 Z71. What all is included in servicing your 4x4. Every once in a while my push button 4x4 slippins out of 2 wheel drive into 4x4 without hitting the button..I once did get a message on my odometer to service my 4x4.  I had trax tire service my 4x4 at 40,000 miles. I don't know exactly what they did, but it cost 385 bucks. My mileage is 99000. Should I have it serviced and what should i expect them to do?
Also, at about 30,000 miles my transmition was clanking going into to gear. I took it back to the dealership and they put a new bell housing (whatever that is) on the transmition. I love my Avalanche, but these transmition issues scare me. As you can tell, I'm not mechanically inclined. Don't plan to get rid of it,  I want to keep it around for pulling my boat. I keep the oil changed and plan to have the trans filters changed and flushed soon. Any thoughts?

I would not sweat it if you don't have to replace the same part twice. The 4WD is probably your encoder motor, it is a very common problem with GM Trucks.

there are 2 ground wires under cab on frame left side clean or repair did the job for me