slip or non slip diff

is there any way to find out if i have no slip on the differential whit out opening it

Four ways actually, one is look at the build sheet from the factory, that will tell you if it was ordered as a factory option. Accelerate from a dead stop into a turn, if one tire breaks loose durning acceleration probably not. Look on the rear end for a metal tag that says 'use friction modifier'. Jack it up and turn one wheel, if the opposite wheel turns in the same direction, maybe, if it turns in the opposite direction then no you don't...
The only surefire, no doubts method is to pull the cover and put your mark 1 mod 0 eyeball on the carrier. At that point you can identify the type if there is one in there. If you do have one and it is not working it is always possible that some previous owner did not add friction modifier when they were supposed to or the clutches are worn out and it needs a rebuild/service.
Bottom line, for a 'beyond a shadow of a doubt' you really have to pop the cover and look.

Get both rear wheeles off the ground,TC in neturl,turn one wheel by hand if other wheel rotates in same direction some type of limited slip is in it.Or there may be a tag on the diff cover telling you to use a friction modifier.
More info can be found if you ues the search button.
Wayne

where is located the build sheet from the factory i have a 90 xj limited

Grab the VIN number off the plate under the windshield.

Call your dealer's service department - they can run it and in a few minutes tell you whether or not it had it when the truck left the factory.

If you've owned it since new, or know for certain nothing has ever actually been added to the rear axle, this may be a simple way to figure it out.

I did it and darned if they weren't right about it...LOL

There may also be a silver sticker in the glove box.