Jeep's are the most wonderful vehicle made for off-road. However, they did not, in earlier years, have that good of luck with the frames. It is interesting to see older Jeeps on the trail; it is amazing how they flex. Unfortunately, that C-channel frame tends to crack too. The '70 CJ5 I used to have had a few welds on the frame.
The Advent of the new CJ7/CJ5 series changed things, unfortunately 30 years late... They went with a full box frame, which is much stronger. They do not flex as much, which helps the frame, although that flex did help off road, trying to keep tires on the ground. The newer CJ7/5/Wrangler family had long springs, which gave more articulation.
The old Willy's probably did ok pulling a plow, since the pressures/strains were in line with the frame. But, as you mention, a boom or a snow plow which is lifted, those would tend to put down force on the frame, causing flex. Too much flex and you wind up with cracks.
Growing up in a CJ2A, we had lots of friends who boxed thier frames in. That made a big difference.