1996 XJ, manual, part-time 4WD, Chrysler 8.25 rear.

Are both diffs limited slip?

 

Most likely neither of them are

 

Then I don't need to add friction modifier when I change the gear oil? Or is that still needed?

 

Front no.
Rear could be. People say that you can see enough of the carrier through the fill hole to find out.
LSD carrier is the first shot. Open carrier is the second.
Also with the rear in the air I think if you spin one tire and both go in the same direction that is an LSD.
Unless somebody welded the rear.
And if they spin opposite it is not.
Hopefully somebody will correct me if I am wrong.

 

Even if the rear has the limited slip the clutches are likely worn out so much that the additive isn't needed.

 

Apparently Mobil1 75W90 already has the modifier in there, so might not need any. Hoping two bottles even will fill it.

 

Two bottles of what size? Because the rear takes about 2.25 quarts and the front I think is around 1.5 quarts or so.

 

Yeah, I was surprised by how much the rear diff needed. I'd get at least 4 of the liter sized bottles for front and back.

 

One of the references I saw said 4 pints for the rear, so that would be 2 quarts. Will see how it goes in a couple of weeks.

 

jack the rear and rotate tyres will tell if you have LSD..I use LSD oil in front & rear anyway, cost about the same

 

Mobil1 LS 75W90 says it is compatible with limited slip, I have a quart of it right now. Apparently the LS in the name means Limited Slip.
https://www.mobil.com/en/industrial/...-lube-ls-75w90