I have a friend with an 05 qc auto 4x4. He has about 17,000 miles on it and has been running an Edge EZ for a while. On 2 occasions now, he will be cruising down the road at 55 or so, let off and the truck drops to 50mph...the he applies the brakes. when he hits the brakes, the whole truck shakes and you can see the tach surge up and down about 500 rpms. its an awful scary feeling, i was with him one day when it happened. it only lasts a second if that long.

any clue as to what this could be?