Front end levels can cause the 4wd shake. Although I haven't heard any complaints from guys with the Leveling struts doesn't mean it's not possible. Most guys with the cheap leveling spacers have the issue.

For the sole reason that your front tires turn when in 2wd, I doubt the wheel spacers are your problem. Most shakes/shimmys when in 4wd are caused by driveline componets that are forced to turn which enhances the feel of the shake. This includes the cv shafts that have u-joints in them. Leveling these trucks changes the u-joint angle and can cause the shaking you are experiencing. I assume you did the proper thing and had an alignment done after installing the leveling struts? If not, highly needed.
I will say that I experience random shaking in my own non-leveled stock truck. Sometimes when I get on the highway it feels like the tires are very out of balance. Then if I slow down and pull over, it stops. I know if I went to the dealer I would be wasting my time and I just accept it for what it is. It's very irritating but only happens here and there. My truck has 10kmi street miles and everything is super tight and not worn.

Good luck and if you figure out the problem post back up with the result!

 

i think its my CV Joints, they need grease since I remembered I had the same exact problem with my 03 BMW 33XI and there was NO grease in the boots, I greased them and it was fine.