Pelican Parts Forums - LS differental or open?

How can I tell if I have a limited slip differential with out opening the transmission?
The reason I ask is while cleaning and checking the inner and outer CV boots, turning the tire while wiping the boots, I notice the other tire is turning the same direction and the same rate. I think, no big deal, the Swepco 201 is causing drag as well as friction on the parts.
Here’s the kicker, I grab the passenger side tire and try to turn the driver side tire, (I can turn the tire 1/8 turn at a time, you know how much room is under there) It took quite a bit of force to make the driver side tire turn. I did the same holding the driver side and turn the passenger side, same thing. When I do get it to break loose, yea, I feel it brake loose as if someone let off the brake. Also, it doesn’t freewheel when it stops, it stops!
I know what you’re thinking, parking brake is off, if I give the tire a good spin weather it be the driver or passenger side, both tires will almost turn a complete 360 degrees so I know my brakes aren’t dragging and causing this.
Who knows maybe I’m a complete wimp and this is a open diff and I cannot get the spiders the walk, but in my Chevy V-8 days I could tell if a diff was open because I could feel the spider gears turning (“differentiating”) while a friction plate type I could feel the plates (“brake free and grab while differentiating”).
It would be cool if I do have a limited slip but if I don’t that’s cool too.
Here is the numbers off of the trans: 74C 1981 X15/6312. The X is either a 0,8, or 9 it's almost ground off and hard to read.