How bad was the vibration? The only time I've experienced tire shake was in my old FED. It rarely shook, but when it did, it wasn't at the hit. It was usually when the front end touched down and it hurt!! I'm talking about teeth rattling, vision blurring shake. It didn't last long (thankfully) but it wasn't just a vibration. Tire shake isn't spinning. It's the tires wadding up and trying to run over themselves. A little spin would be much better.
I'd think with that much torque and a leaf spring suspension, you really need more down angle on the pinion. Probably to the tune of 4-7 degrees down from the driveshaft at rest to be straight under power....