Mine is doing the same actually, and I can't figure it out. The diff is full, and I've got Mile Marker hubs - at certain speeds it isn't noticeable, or I should say at certain power? If I let off the throttle it'll make the growl and if I get into it it will do the same. I did balljoints, tie rods, seals, etc probably 20K ago and at that time the front U-Joints were both tight as could be. I'm almost wondering if it isn't something to do with the T-case, but I'm not sure - I've ran it as is in some crappy weather in 4wd and although loud and making noise it seems to be working properly? I sure would like to figure out what the hell it is, because it is rather annoying and being that I commute 320 miles each way for work several times a week in Montana/ND winter I would like to not have the worry of it taking a **** on me in the middle of nowhere.