I recently attempted to grill duck legs on my propane Webber. I was afraid of flare-ups due to the high fat content in the duck meat so I grilled with somewhat low and indirect heat. It took a long time, but I got them looking lovely and brown and not burned. The only problem was this: they were tough and didn't taste very good at all. Clearly I did something very wrong. Any advice?
I think grilling is probably a bad plan for duck legs; the fat content is a real danger like you said, and duck legs are tough enough you probably want to confit them or braise them.
If you absolutely have to grill them, I would suggest confiting them at 200 degrees for three or four hours first (you could use veggie oil in a pinch) and then resting them in the fridge for a day or so in oil.
As for finishing them on the grill, rinse them off gently, re-season if needed, cook flesh side down on a medium heat portion of the grill for a while until mostly heated through, then flip them over on a high heat portion of the grill to crisp up the skin, watching out for flares.
To grill a duck.
Poke through the skin at 1 inch intervals so that the fat can drain easily. Use a drip pan filled with water directly underneath the duck to prevent flareups (make sure fire is not directly underneath the drip pan). Roast at 325 for 2-3 hours. Make sure internal temp is at least 165. Skin should be thin and crispy.