You might be surprised at the speeds when shaking will happen. Hypothetically, it would seem logical that a vibration, once it starts, will only continue to get worse as speed increases, right?
Quick story...I damaged my driveline recently in Hollister and it wasn't real bad but I managed to drive home the 90 minutes or so. Well, it started vibrating at about 40MPH and got progressively worsened to about 50MPH. at around 55 it started to go away and at 60-65 you couldn't even feel it!
Yeah, no lie!
You might even have a u-joint that is siezing and not turning smoothly which may cause you issues at speed and won't feel loose if you grab the driveline and shake/move it. You might even look to see if the driveline is phased correctly (Yokes positioned the same at both ends) this could cause a vibration too. All good things to check. Basics!
