If using bamboo skewers, soak in water for at least 30 minutes.
In a medium glass or stainless steel bowl whisk the aioli ingredients. Keep chilled until ready to use. (The aioli may be prepared one day in advance.)
Prepare the grill for direct cooking over medium heat (350° to 450°F).
In a large bowl combine the skewer ingredients and toss gently. Thread the steak and zucchini pieces on their own skewers, making sure to skewer the zucchini through the peel so that as much surface area as possible will be touching the grill. Do not crowd the ingredients.
Brush the cooking grates clean. Grill the steak kabobs over direct medium heat, with the lid closed as much as possible, until nicely charred, 4 to 6 minutes, turning once or twice (if flare-ups occur, move the kabobs temporarily over indirect heat). Grill the zucchini kabobs at the same time over direct medium heat until nicely charred and crisp-tender, 4 to 6 minutes, turning once or twice. Remove from the grill and let rest for 3 to 5 minutes. Serve warm with the aioli.