axle bearings.. spiders on a wrangler on 31's (blown 31, replaced with DONUT from cherokee !!! bad idea.. increases spider rotations and chunk.. lost tooth.. lost two teeth.. stripped!, replaced spiders + tire and rolled on for another 30k+ [sold]) aside from axle bearings and that one set of spiders, no broken shafts, current is on 33's with '88XJ 35 non-C-clip [due for 8.8 cause axle bearings are raising cain, but still going]
my older '91 XJ had the '35 and it never did break and I beat it [on 31x10.5's], loaded it up (40 40# bags of concrete!! tires+springs didnt like it)
my other 91 XJ had the '35 never broke either, had 235's then 33's, then swapped for 8.25 because of bearing noise and I had the 8.25
mostly backlash though is the problem, rolls about a foot when parking! i bet it would last a good while if i stuck bearings in it and fixed the backlash. backlash = sheared teeth on grabbing traction. oh and I never did really baby them on/off asphalt [weight load or skinny pedal load].
all 3 d35's geared 3.55, open