This is my first post.I have a 06 Screw with 20 in Pirelli tires that shimmie a little at 60 to 70 mph.I looked on this forum and found a sevrice bulletin # 18891.My question has anyone dealt with this?

 

I highly suggest you get all 4 wheels spin balanced on the vehicle (you will need to repeat this with every tire rotation). With that tire/wheel set up, proper balancing is more critical. However, that TSB sounds as if the tires are simply "flat spotting" when cold. Your running low profile tires on a heavy vehicle.. your gonna feel a lot more road and vibration with that set up as you described.