In a medium bowl, whisk together the soy sauce, orange juice, pineapple juice, brown sugar, ginger and garlic. Place the pork tenderloins in a ziploc bag and pour the marinade over the pork. Seal the bag and refrigerate the pork for 8-12 hours (perfect to prepare overnight for grilling the next day).
Preheat an outdoor grill to medium heat. Grill the pork tenderloins for 8-12 minutes per side, until the pork is cooked through. Tent the pork with foil and let it rest 5-10 minutes before cutting into slices and serving warm.
Notes
Ginger: fresh ginger is very easy to use. Simply peel the skin from the knobby piece of ginger and then grate the ginger on the small holes of a box grater or on a rasp grater. I haven't tried this recipe with ground ginger so if you prefer to use that, you'll have to play around with the amount.