Mix together the salt, garlic salt, celery salt, black pepper, onion powder, paprika, dill, sage, and rosemary in a bowl. Store in an airtight container at room temperature until ready to use.
Use a damp towel to lightly moisten the roast with water, then pat with the prepared rub. Refrigerate for a minimum of 2 hours, up to overnight, for the flavors to fully come together.
Preheat an outdoor grill for high heat and lightly oil grate.
Place the roast onto the preheated grill and quickly cook until brown on all sides to sear the meat, then remove. Reset the grill for medium-low indirect heat (if using charcoal, move coals to the outside edges of the grill pit).
Return the roast to the grill, and cook, turning occasionally, until the desired degree of doneness has been reached, about 1 1/2 hours for medium-well. Remove from the grill and cover with aluminum foil. Allow to rest for 10 minutes before carving across the grain in thin slices to serve.
