I wouldn't rely on a differential tag being 100% right. They may have only looked at that and not the internals. You could always do the old school method of jacking up the rear end, putting the car in neutral, and spinning the tires by hand to see how many revolutions the driveshaft makes in relation to one tire rotation. (If your car has a limited slip, both tires should spin the same direction, and you only need to spin them one full rotation. If you have an open differential, one tire will spin forward and the other backward. If it's an open diff, leave one rear tire on the ground. Turn the tire that's off of the ground two full rotations, not one.) Mark a line on the tire you are spinning, and one on the driveshaft.
So, Limited slip: Both wheels off the ground, turn one complete rotation and count driveshaft rotations.
Open Diff: Lift one wheel off the ground and turn 2 complete rotations, then count driveshaft rotations.
It might be a bit hard to count the rotations. I usually make chalk marks to line things up with. If your driveshaft turns roughly 3 and a quarter turns, that should be 3.27s. If it only spins 2 and a quarter turns, sounds like 2.73s.