Question about Differentials
 

Which diffs are widely considered to be the best for front and back axles?
I'm looking into adding another XJ to my collection and there just so happens to be a very (imo) nice looking XJ in my area and it's 4WD.
My current XJ is 2WD and assuming everything is well with the one I'm considering I'll be donating the 2wd to my mom as a birthday present cause she always wanted an XJ :thumbup:

TL;DR Best front and back differential for a 4.0 Automatic 99' XJ Classic?

The Chrysler 8" Diff is superior to the Dana 35. D35 on many older, newer XJ's had the Chrysler rear end. Your 99 models in consideration will likely have the Chrysler rear. Parts supplies are plentiful and the rear end was often used in the Liberty model.
The D44 is next step again above the Chrysler diff, but rare in comparison.
D30 front Diff is pretty much the standard, D44 would be superior but rare. The D30 can be upgraded to survive.
Survival rates are really based on off road activity and increasing tire sizes (hence increase stress loads on the diff).
I have the stock D35 in our 89XJ with 755+Kkm on it. Stock main bearings, replace outer bearings and seals about every 150 - 200 Kkm. Primary roads, trails, malls, not seriously off roaded.
If you are picking up the 4x4 XJ primarily for off roading, you will want to upgrade both front and rear at some time to be able to survive the large tire sizes that will be in your future.
$$ spent will depend on your objectives.

For tire size, I don't foresee going more than a 32x11.50, and as for offroad since it would be my new DD it would likely see more or less 60%-70% pavement and the rest being mud and light trails. I'm not very interested in rock crawling. If worst case scenario is a D35 in rear and D30 up front, would it be pertinent that I get one or both replaced?

The d30 in the front will do you just fine. The c8.25 is definitely an upgrade to the d35, but for what your intended use is it'd serve its purpose. I would not consider it pertinent to replace the d35, but if the option is readily available it wouldn't be for nothing.

Sounds good, thank you!

Consider searching for an 8.25. Run the D35 till it breaks, then swap. Could be a bit of challenge to get off the trails with a broke D35.
For longevity, don't get rammy and jammy when stuck and keep the tire size 32" max.

No Rammy Jammy is more fun than Breakie!
My XJ has been rammed and jammed, on occasion, with the stock D35 (89 type, non C Clip) (stock but regularly maintained with fluid, bearing and seals changes), and has 755+K km on it (470K mi).
If I break and axle the outer plates and bearing keeper ring will resist the axle coming out as we snap crackle and pop back the trail head. The new D35 with C Clips axle shafts will want to walk out of the axle housing after breaking. (1 minute rammi jammi = 30 min trail ride turns in 4.5 hrs of YJ recovery fun, wet, cold, muddy) Fun Wow how do I know? However, no Jeep left behind.
If you have an opportunity for an axle system upgrade for a good price, I recommend the purchase.
For primary daily driver, keep your D35 and maintain it.