Pierce roast at 1-inch intervals with fork; set aside. (Piercing allows marinade to penetrate the meat better.)
Combine salad dressing and remaining ingredients in a large heavy-duty zip-top plastic bag. Reserve 1/2 cup mixture in refrigerator for basting during grilling. Add roast to remaining mixture in bag; seal bag, and chill 8 hours, turning bag over occasionally.
Remove roast from marinade, discarding marinade.
Coat a grill rack with cooking spray, and place over medium-hot coals (350° to 400°); place roast on rack. Cook, covered with grill lid, 35 minutes or until meat thermometer inserted into thickest portion reaches 160°, turning and basting with reserved 1/2 cup dressing mixture after 20 minutes.