.Being in australia GMC knowledge is rare as hens teeth. When in reverse it will idle back but sometimes when any acceleration is given a loud ( clang ) is heard and the 4x4 stick jumps to neutral while in 2WD Sometimes it clangs the stick moves but stays in 2WD. any help on this would be GREAT.

Could be a broken snap ring on the output shaft (see http://www.drivetrain.com/gillies/Tcasesaver.html)

//2000CAYukon

You can get an upgraded snap ring which is better than the factory for the output shaft of the transfercase.

Thanks very much the suggested parts are now ordered,