I own a 2004 F-150 Heritage, with 4x4 shift on the floor (vacuum) when i put it into 4 wheel drive and step on the gas moderatly or while turning the front axle bangs and disengauges from 4 wheel drive. If I drive straight or am easy on the gas it will stay in 4 wheel drive most of the time. Any one have an idea on what to check.

John