And I've never actually seen one of these axles break even with 35's.
I have... I've seen plenty of them break with even 33" tires (and they weren't the very weak EB D30 like what you have, but rather the "better" HP Jeep D30s).
If you plan to keep small 235/75R15 tires or something similar size, then you may be able to get that D30 to hold up reasonably OK. But forget about big tires (problem is the weak LP ring & pinion gears, there's little you can do to strengthen them other than maybe doing a cryogenic treatment on them, but IMO, it's just not worth it on that axle since the housings are not all that strong either).
Sorry to be a bearer of bad news, but there's a reason people get rid of those for D44s or better (it's quite very possible the axle you currently have under your truck now is worlds better than that D30 too, so you're probably better off just leaving things alone for now).
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