Welcome...I noticed it on the 405 as well at the 680 in San Jose in my CM. I dont know if it is less in a CM. It may be different from the difference in weight distribution due to the cab and/or the extra foot of bed space.
I have a 6" Pro comp lift and still feel it but not that much in the cab...more looking in the rear view mirror and seeing the bed vertically going up and down. Gotta admit I was worried it would hit the cab.
Dont worry...truck is worth it. There is a thread somewhere on this forum talking about stuffing a rag somewhere in the rear end to get rid of it and change the harmonics but you'll have to search.