98 4×4 2.7 liter 5spd 160k miles. when driving in 3rd and 4th gear my truck starts shaking i can feel it in the steering wheel and see why review shaking only when im going beetween 35 and 45 mph though and only in 3 and 4th gear. shifts perfect and drives perfect in 1st 2nd and 5th gear . also drives fine in 3rd and 4th gear except when going 35-45mph. what could it be. no power is being lossed drives fine any suggestions?