I've done a few leg of lamb an two sets of rack of lamb..
I'm no expert by any means but here's what I did..
I did a marinade for 18hrs. APPLE JUICE, APPLE CIDAR VINEGAR, BROWN SUGAR, SALT, PEPPER, GARLIC, BAY LEAVES, ORANGE SLICES, AND ONION..
Before setting on the grill the morning before my cook.. I rubbed it down with kosher salt, pepper, garlic powder, cumin, thyme, and a lil more brown sugar..
I did on a weber over direct heat about 375-425 for an hr or lil more gave it a good char.. I moved it over to a taller grate pit an ran about 300 for 2 hrs then wrapped in foil for the last hr..
To say it was GREAT would be an understatement honestly.. The bones on the rack just slipped right out when pulled.. The leg had a finishing taste an texture very similar to barbacoa ( cows head/cows tongue ) it was delish an every time it was made there were NO LEFTOVERS
Good luck an I hope this helps a lil