Pop the diff cover off, and change the fluid at the same time.
Also could just light the tires up on a dirt road, see if there is two trails or one

 

yea it totaled but i bought it back from the insurance and im rebuilding it...

 

That's cool....looks like a good looking rig

 

Jack the rear end up and spin one wheel (in neutral obviously). If the other spins in the opposite direction, you have an open differential.

 

Like everyone said - Tough question;
My plan was to put a Spatran in the front & stick with my LSD (limited Slip Diff) in the rear (8.25). Plans changed, Since I live in Buffalo NY & we get tons of snow, I'm going with a Air Zip Locker in the front & staying with the LSD in the rear. The LSD won't handle as good as a locker off road, but it will have great road handling. The Spartan IMO is the wrong choice for me, that's why Im going with the Selectable locker up front.

LSD not too easy to notice if you pull the diff cover, not impossible. Here is my out of my 8.25 (stock from the factory)

Look close you can see the clutch plates:


Lunch Box Lockers: I had a LockRite in the rear of my TJ, hated it. My buddy had one in the rear of his XJ, blew it up. I've heard nothing but good thing about Spartan & Aussi.

Here is my Spartan, wanna buy it. New in packaging:

 

Thanks for the ton of info you guys put on here. Sounds like if I go with a locker it will start in the rear brand unknown.....maybe a used one if I can find it.

 

Not like we have much snow now, though driving yesterday was fun. Where abouts are you in B. Lo? Back on topic; get a locker that fits how you drive and your budget. Next week I'm swapping in an 8.8 with an Aussie locker and will be putting in a Yukon Zip up front.

 

I'm in Hamburg, & ya, went wheelin' around town in the snow after work yesterday after i plow'd my driveway. Told the wife I was gonna get gas & was gone for an hour or so. Did you buy your Zip yet? where you gonna get it from? I've been PM'n HWK for gears & locker? I'm workin on swappin in a HP30 & 8.25 to get rid of my LP & Tirdy5!

 

Pm sent as to not clog thread!

 

Though for those of you looking into gears and lockers, RWK is a good guy to deal with and I'll be placing my order for a Zip and 4.88 gears as soon as funds allow.

 

I just did the trail that is in your video a week ago, was easy but alot of fun. This picture was taken in that exact same spot.

 

Nice....yes its a very fun trail. I have lived here my whole life and I have watched this trail deteriorate more and more every year. Makes it fun though I just hope the fs don't close it.