Chrysler DID update the 8.75 from the small pinion rear that it was, to the larger unit for its hi-po cars, but obviously they didn't feel it stout enough for big block and HEMI cars, so they sourced DANA to build a pass car model. Ford on the other hand, felt their 9" WAS strong enough for their hi-po cars with a few upgrades, so they came out with the 31 spline, locker, nodular case, with better pinion support model, which WAS an extremely stout factory rear.

All 8.75s are not created the same and neither are 9" rears. While run of the mill 9s were weak, the hi-po model that came under super cars and some trucks and vans, was NOT. Same with 8.75s. The small pinion A-body type 8.75 is weak as water, but the larger pinion models are MUCH stronger, but still not nearly as strong as the BEST 9" rear. You have to compare apples to apples.

So, is the 9" as good and heavy as a DANA for a heavy duty truck app..........NO, because it was not designed to be that.
so they tooled up to make everything else better? fixed everything but the housing