You have an open. Most all 7.5's do.
If it were a limited slip, it would say 3L73 instead of 3 73 as it does on your tag. You have 3.73 gearing as your already figured out.
Some 8.8's that had open axle tags actually had LS diffs -- I had a friend with a new truck, he was the original owner, and when we went to put a NoSlip lunchbox locker in, it was the wrong one because he had an LS instead of an open! I've heard of this happening a bunch of times now so Ford must have run out of open diffs or something and put the LS in and used the originally intended tagging.
I've never seen a 7.5 with a LS, actually -- though they probably exist.