7 1/4 why weak?

I've got a pile of seven busted 7-1/4 rears behind my Dad's garage from various vehicles I've owned. The spider gears and shafts are broken out of the case on each of them. The most powerful engine that broke them was a 273 2bbl. What kills them is when one wheel is spinning and then grabs. They are too small for even a slant six car if you live where there is ice and snow. I've got to admit that I did break a couple by being a bit aggressive but the others just failed in normal driving. (of course several of them probably had 100,000 miles on 'em by the time I got the cars)