I have a 95 GMC 1500 Sierra 4x4 5.7 auto with a problem when reversing in 2WD . It idles back ok but if the excelerator is used the 4x4 stick oftens jumps to neutral with a ( CLANG ) Any ideas? GMC knowledge in Australia is as rare as hens teeth!!! It changes smoothly though all forward gears. It has 195000 Klm on the clock.