I don't really marinade stuff much anymore - while it does add flavor, I never thought it did that much for juiciness (at least for big cuts).
You might want to try brining the pork. Add a cup of salt to a gallon of water and let the meat soak for a few hours/overnight. It's easy to add other things to add flavor, like brown sugar, molasses, bay leaves, garlic, cloves, soy sauce, ginger, thyme etc. One of the things I like using is a few bags of herbal or fruit tea - apple cinnamon goes well with pig. Just remember to give it a rinse before grilling so it's not too salty.
Edit: That's a cup of kosher salt, if you're using table salt, cut to down to 2/3 a cup.