What is your setup on your truck?
I say run Spicer 5-760x ujoints. They are the strongest factory ujoint on the market beside the custom stuff (CTM, OX, etc). As long as you aren't abusing your truck you should be ok. The cost of a CTM or OX is not justified if you are running stock axleshaft.
I am running 35" tires and I rock crawl my K5 and they are holding up fine for me sofar (knock on wood). I have a lightish driving style though. I have seen 760's break on a truck with 36" tires and a heavier driving style in the rocks.
Harley