My 97 Sierra has developed a problem! About 4 times over the last 6 months when I am backing up it seems to drop into park( or at least the parking detent seems to engage). It sounds just like you put it in park when it was still moving, though the column shifter never moves. It only happens in reverse, but I'd like to figure out what is going on before my tranny completly fails.
Has anyone heard of this before ? Sure appreciate any help.

Bob

check your linkage setting on the tranny, some older trucks do this alot if it has had the tranny worked on, simple way to check is put the park in neutral get under the truck, and unhook the linkage (block the tires first) turn the linkage by hand, 2 clicks hits park, 3 clicks is drive and low gears, set the truck in reverse, get out, place the selector in reverse and hook everything back up. Might help and worth a try.