Place pork shoulder roast in a clear plastic bag; set in a deep bowl.
Thoroughly blend together the soy sauce, dry sherry, minced garlic, dry mustard, ginger and thyme. Pour marinade over meat in bag; close. Place the roast in the refrigerator and marinate for 2 or 3 hours or overnight.
Transfer the pork roast and marinade to a crockpot. Cover and cook on HIGH for 3 1/2 to 4 hours.