I have a 2007 Ford Ranger XLT 4.0L extended cab with an automatic transmission. I was driving on the freeway New Years eve in a snow storm. The roads were bad so I engaged my 4X4. I noticed every so often my truck would hop like I was hitting a bump in the road. It happened about every 25 seconds at 60 mph. I had my girlfriend time it. I disengaged the 4X4 and it stopped. When I engaged the 4X4 again it started doing the same thing. What could be causing this? At about 2000 RPMs every 25-30 seconds at 60 mph it would be hapening every half mile or at every 1000th rotation. It seems that if it were mechanical it would hop at much faster intervals. So im thinking maybe something electronic. Any thoughts?