4WD "bumping" problem
When in 4WD, my auto trans '03 Ranger makes a "bump" sound. The whole vehicle jerks when it happens. Dealership put 4 new tires on it and replaced the drive shaft with no improvement. "Bumping" is louder and harder when decelerating and does not occur when accelerating. Ranger has 28,000 miles on it. "Bumping" is most noticeable on dry pavement but occurs on other surfaces also (including gravel and snow). When switching back to 2WD, one last "bump" signifies the disengaging of the 4WD. Dealership techs have never seen this and don't know what to do. Help!
PS-Bumping happens every 1/8 mile or so when driving steady. When decelerating, bumping happens every few seconds.