Truck Forums - help please

So i have a 2005 gmc 1500 4x4, with around 140k miles, and i started hearing a faint ticking or ratcheting sound coming from the undercarriage while im in motion, usually when i drive next to a building or bridge, i can hear the sound bouncing off the structure. well, now randomly, sometimes 4 times in a mile, sometimes not for a day or so, it will completely kick out of gear. took it over to a mechanic friend of mine, he dropped the transfer case, and found a nut, i belive on the chain tensioner in the TC, had backed off. he replaced it, and that was that. i left his house and got 30 feet down the road, and it kicked out of gear, there were no gears. we pushed the truck in park! we took out the 4x4 fuse, i put it in nuetral, pressed the 4h button, then the 2wd button, and it drove normally. every now and then it kicks out, i pull over, put it in n, and repeat the process. its my only mode of transportation, and being a disabled vet going to school, i dont have the money to get another vehicle. My mechanic friend is stumped, and frankly, so am i. so i figured id get on the forums and see if anybody else knew what to do.