A bit over 11 inches are 90mm, and 12.5 inches are the 100mm.
Now here is another trick, most auto parts stores won't list the axles as 100, and 90mm. They list the diameters in inches. 3.5 are 90mm and 4.0 are 100mm
Your original TJ differential came with 90mm cups so they have been changed..
The easiest way I found is to look at the bolt pattern, if all the bolt holes are equidastant, then 90.
If the bolt holes are off set, then 100.
I had 100mm axles and 90mm cups on my TJ tranny. So I swapped the axles out to have a spare set of 100mm axles, and picked up 2 new 90mm ones.
You can swap the cups out easily enough, and I had to throw a TNA differential back on the car when the axle broke and beat the hell out of my differential till it gave up. So I swapped the axle cups again... no biggie.