I would first try to fix it yourself. You should immediately check the torque on each of the 10 mm nuts holding the third member to the axle. If any are loose, tighten to spec torque and see if that stops the leak. If not, proceed by draining the rear diff fluid and removing the third member. You'll have to remove both rear wheel speed sensors and slip the axles out of the differential a few inches (be careful not to damage the axle seals), then remove the third member. At that point you can use Rv blue sealant or the toyota paper gasket, reinstall the third member and torque all nuts (24 ft pound, but check that #), refill the fluid, and you should be good to go. No idea what labor would be to do this for you. If you get a factory manual and follow all instructions this job should be a breeze.
