Well, to be completely honest, there are two vibrations. If I get on the gas from a standstill, there is a prominent knocking right where the transfer case is. If I don't stand on the gas, it doesn't do it. So I would say it is speed related, if I am even going 10 mph, and I stand on the gas, it will not do it. When it switches into second, it shimmys too, but nearly as bad, it is just a vibration in second. The vibration in first is like I ran over grandma and she is knocking on the floorboard!
Don't take life to seriously, nobody gets out alive.