For most trails, the stock 27 spline axle shafts will hold up to 35" tires and an automatic locker. This is of course providing you stay off the gas when the steering wheel is cranked over which is when the u-joints & ears that hold them are under the greatest stress.
However, if you do pretty tough trails, it's a good idea to upgrade the front axle shafts with hardened 27 spline alloy axle shafts like from Superior Axle. Once those are installed, it's pretty well "bulletproof". Plus by going with 27 spline alloy shafts, you can still keep & use your original OE 27 spline axle shafts as spares, which I doubt you'd need once the hardened alloy axleshafts are in. You could not use your OE axle shafts as spares if you went with a Super 30 kit which uses 30 spline axle shafts.
I broke a front OE D30 axle shaft on Sledgehammer in Johnson Valley years ago on my locked axle with 35" tires. But after upgrading to Warn's hardened 27 spline alloy front axle shafts & hubs, no more problems. Most of the guys I wheel with out there with Dana 30 axles just went with alloy 27 spline shafts which is a good match for the Dana 30's overall strength. My front axle has been very reliable since installing the Warn hardened 27 spline shafts. I personally would not install a Super 30, I'd rather be able to use easily found or existing 27 spline parts as spares.