i need some help don't know what it is , when im driving on the freeway my truck jerks from what seems to be the front all the way to the rear end like its kicking back and it does it only when im on the freeway i have replaced the plugs cap rotor thinking it was miss firing but no luck i also have a new belt on it i think its the belt tensioner but not to sure any suggestions.
my specs are 4.3 vortec

 

my blazer jerks also, but unlike yours, its shifting from first to second, i have an auto tranny and i too have replaces the plugs and cap. anyone with help...help!

 

yea i think i may have it narrowed down to the clutch fan it seems to be overexcessivly be in use like fully locked in so i am going to try that next and see what happens