My engine grunts louder too at that speed. It lasts from about 3000-3500 RPM then it quiets down again. I figured it had soemthing to do with intake. exhaust, or harmonic balance, but if it's a 4WD thing, then I have no idea. I wouldn't think the extra weight would cause that but perhaps different routing of the exhaust (is it different???) could cause frequency changes. Beats me!