I have a hummer that i have replaced transfercase with used out of salvage yard but didnt fix the problem but it will drive fine until at cruising speed and all the sudden kicks out and wont move it makes a little bit of a quiet moan noise but i crawled underneath and the front driveshaft spins but the rear doesnt and none of the tires move. i started to pull it back to shop and when the speeds were met then it kicked back in and it drives fine but much over 30 mph and leveled out speed it kicks out. i cannot figure out what is going on. plastic shift fork in t-case looks normal and fine. any ideas? you can hear it switch from 4 hi to 4 lo and back to all wheel.