why does my truck not want to stay in (P)PARK position when i shift to it? on 2002 Chevrolet Avalanche 1500

when i put it into the (P)PARK position it still moves and doesn't want to lock just keeps moving, and when i go about 5-10 mph the truck jerks forward every few seconds, got about 140,000 miles on it, don't know if the oil was changed in the transfer case, idk what's going on, first time that has happened

1 answer
the shift cable may need to be adjusted. let a trans shop check it for you to verify.

Roy