While grill is heating, prepare seasoning paste and apply evenly to rib roast. Let roast stand at room temperature for 15-20 minutes before placing onto grill.
Add pre soaked wood chips to grill according to manufacturer's recommendations. Place rib roast on grill rack. Be sure to have an aluminum drip pan filled with an inch of water underneath. Cook for 2 hours or more, adding fresh wood chips every 30 minutes,
Once done, remove roast from grill and let stand (loosely tented with aluminum foil) for 10-20 minutes before carving.
While roast is resting prepare sauce. Combine sour cream, 1/4 cup Dijon mustard, milk, green onion, and ranch dressing in a bowl. Serve on top of sliced rib roast or on the side.