I have a 99 7.3 f350, started shaking really bad at around 1200-1500 rpm’s and sounded like it was misfiring. after 1500 and below 1200 it would run fine. So as anyone would think maybe the injectors? But no smoke or problems starting… seen a lot of people with this same issue on these forums but no one has said what they found to fix it. But my problem was actually the transmission bolts! Who would’ve thought lol. They backed out a tad and shook the whole truck. Tightened them back up and it’s perfect now. Hopefully this helps out all the people who has found nothing. Maybe it’s your problem, maybe it’s not. But it’s worth checking before you spend $$ on other things and then next thing you know your transmission goes flying out while going down the highway lol.