Start with basics: look for anything loose, worn, feel for any type of wear on the rotors. Give them a good cleaning and then go for a spin. It may be as simple as you've got some gunk in there somewhere and it's just making noise.
If you want to check the driveline, take off your rear driveshaft (easy) and put it in 4HI, go for a short drive around the block and listen to see if the grinding is gone. You could do the same for your front shaft just to see if that could be doing it.
Have you checked the wheel bearings yet?
Since you have a late model and no SYE or t-case drop, it's very possible that it could be your driveline. The only thing that makes no sense is how you say it does it in "bursts". Just speaking from personal experience and also hearing it on a friends Jeep, ours just did one quick, loud humming vibration. After t-case drops (we're both on lower lifts than you) they stopped. Again, just what I've seen and doesn't discount your issue, but the fact that it seems to do it in rotations on yours is a bit puzzling.
