[blazerholic]

U-joints, splines where 2 halves of driveshaft go together, pinion bearing maybe. I'm just thinkin it must be something worn that's causing the shake while under torque but not while coasting. Of course, I've had some weird experiences with tire shake before too. What size tires are you running? Axle/wheel bearings could be a factor as well. Hell, just replace everything from the frame down!