I am wondering if anybody on here is running a doubler kit on an xj with the np 231. I know the 35 in the back wont hold up so im swaping in a 8.8 and im hoping the front dana 30 will hold up with 4.10 gears and 33's. Also will the back np231 foront and rear output hold up to a 7.3 to 1 output. I know a dana 300 are atlas will be better off but I already have two np231 and one is ready for hacking up....

there was a thread on this a couple days ago,, look around i know it here somewhere

Search for "rockcontrolXJ"... he has a doubler on his XJ... he also did a side by side of the two avaliable 231 crawlbox kits

ive got a 231/300 homebrew in mine it works very well allkindsa gear choices. As for strength issues with a d30/8.8 there are prob. many other weak links but run it till it breaks then upgrade what ever fails right? There isno solid answer to what is "strong enough" because everybody drives differenly along with amillion intangibles. I have wheeled a stock (flipped) 300 behimd a231 range box for over a year i have a 4.0 and auto no failures of either tcase does that mean it'll work for you? nope but not abad place to start

I was thinking about a 231-231 doubler in my rig. However with the amount of torque the rear case will see, I decided to just do a 231-300 doubler.

I will eventually be doing the 231 crawlbox in my ZJ. For the 231 crawl box I will be getting in the 241's 6 planteries instead of the 231's 3. And my existing 231 is already built to the hilt (6 planteries, wide chain, 32 spline)

Away from the doubler and to your axle question:

I run a D30 and 8.8 on 33's. Its a good settup. The D30 is a decent axle for mild to moderate wheeling.

My d30 was locked on 33s, never a problem.
Nexy year it'll be locked on 35s.