Help !! I have a 1999 2dr tahoe 4x4 a week or 2 ago it's stared shaking at 2k rpm 30 mph in 3rd and 4th gear. I thought maybe a misfire so I changed spark plugs ignition cap and rotor and wires. Stared right up sounded good but same problem. So got up to 30 mph and put it in neutral no more shake. Put back in drive shakes put it in 4x4 and it still shaked but not as much any idea what it could be?