Prepare an ice water bath in a large bowl or pot. Set aside.
Bring several inches of salted water to a boil over high heat in a large saucepan or stockpot. Add the lobster tails, cover, and blanch for 1 to 2 minutes. Transfer the tails to the ice water bath and let cool. Drain thoroughly, then cut out the soft "belly shell" with a kitchen shears or clean pair of tin snips.
Remove the tail meat from the shells and set the shells aside. Remove the intestinal tract from the top of each piece of tail meat. Chop the meat into bite-size pieces and combine with the crabmeat or shrimp. Set aside.
Make the stuffing: Melt the butter in a large skillet over medium heat. Add the onion and garlic and saut until soft and translucent. Stir in the parsley, tarragon, if using, Old Bay, and fresh lemon juice. Remove the skillet from the heat and let cool. Stir in the reserved tail meat and crabmeat or shrimp. Add the cracker crumbs and gently stir to combine. (Do not add the crackers until you are ready to cook or they will get soggy.)
Stuff the reserved tail shells with the stuffing, mounding it generously, and place them on a rimmed baking sheet. If the shells are "tippy," use crumbled aluminum foil to prop the shells upright.
When ready to cook, start the Traeger on Smoke with the lid open until the fire is established (4 to 5 minutes). Set the temperature to 425F and preheat, lid closed, for 10 to 15 minutes.
Put the baking sheet with the stuffed tails on the grill grate and roast until the stuffing is crispy and golden brown, 15 to 20 minutes. Serve with lemon wedges.