Combine all of the rub ingredients in a small bowl, then rub over the meat and let it sit out for 1 hour, covered, so it isn't cold when it goes on the grill.

Preheat the grill for both direct and indirect heat.  Sear the tri-tip over direct heat (right over the flames) on both sides, about 5 minutes per side.  Transfer to indirect heat (between 250-300 degrees F) and cover the grill, letting the tri-tip cook until it reaches 130 degrees F on an instant read digital meat thermometer inserted into the thickest part of the meat for a medium-rare tri-tip.  This should take roughly 20 to 30 minutes.  Remember that the temperature will continue to rise about 5 degrees after taking it off the grill. 

Let the tri-tip rest for 10 minutes before slicing it in half to separate the two different grains, then slicing thin slices against the grain of each piece.