I have searched long and far for a solution to my issue but have found little to no help so im hoping by posting here someone will have some suggestions. My 2008 sierra 2500 SLT has always had a vibration on some level for its entire life. It was my dads truck from 2008 to 2016 then I bought it from him and have also inherited this problem. the shaking feels like an up and down motion causing my passenger seat to visibly shake pretty heavily. I can feel it in my seat as well as the center console. I recently got new tires put on and the vibration got 10 times worse so naturally I went back and the shop re-balanced everything and said some steering components are worn and that is the cause of my issue. On my drive home the vibration felt a little better but still heavy and very much still there. I then had the tie rods inner and outer replaced, as well as the idler arm, idler bracket and Pitman arm replaced. These repairs did seem to reduce the vibration even more and after a 3rd alignment and rebalance the issue is still present but less severe than it was initially after the new tires. The strange part is the vibration seems to come and go and it is not connected to accelerating or braking. I will start my drive without issue and after cruising at 65-70 mph for a 5-10 mins the vibration will slowly begin until it is heavy and really bothersome. But then after a few more minutes it will dissipate rather quickly for 30 seconds to a minute until returning to the full vibration. this heavy vibration is pretty much noticeable over 45 mph and gets worse with speed but is always cyclical, coming and going for no reason and for seemingly random lengths of time. I should also say that at about 20-30 mph I can feel the truck going up and down like my tires are egg shaped but assuming this was the cause of my issues I would think the vibration would be present all the time and not come and go as it does. The tire shop allegedly checked the rims and tires for roundness and damage and I did mention the egg thing to them to which they had no answer claiming all tires are balance properly. In terms of diagnosis I have tried putting the truck into neutral at highway speed to see if it is engine or mount related but it has no effect. I have checked U joints and everything appears good. I am going to put the truck on stands and see if I can get the vibration to show then inspect and remove parts until it stops but I was really hoping someone else has had this issue. It is starting to get unbearable! Thank you for any advice or thoughts on this.