That's a pretty common weak link, even in stock form. The front closed knuckle Dana 60 in the M715 is somewhere around the same strength as a Dana 44, give or take a bit.
Somewhere, and I am sure Jon knows where, there is a link to a website where a guy was modding the closed knuckle 60 with heavier shafts, but if I recall correctly, it took a lot of machining and mods to get anything else in there. Seems like the axle tube and knuckle were the limitations.
"Free advice is worth what you pay for it."™