First, 99.99% of any GL-5 or GL-6 gear lubes you find on the store shelves will already have the friction modifier additive a clutch-based limited slip differential requires. So whether you have a limited slip diff or not, you will likely be ok with any good quality GL-5 or GL-6 gear lube you find. The rear label will have words something like "Compatible with limited slip differentials" if it has the required friction modifier additive. Again, odds are it will have the additive so you should not add more.
One way to tell if you have it is to jack up the rear axle so both tires are off the ground. After shifting the transfer case into Neutral, rotate one of the rear tires with your hand. If the other tire spins in the same direction, a limited slip differential (or locker) is present. If the other tire spins in the opposite direction, it's an open differential... or the clutch pack in the Tracloc limited slip differential is completely worn out and needs to be replaced.