Sorry, was in line waiting and didn't have time to finish this thought to visually show what Bryan was saying...
First, am NOT a fan of IRS for towing heavy. Think the OEM/designers sized it for the +90% who do NOT tow heavy, so the question is: are the precision bushings/etc sized for towing heavy and for the out years ?
Kinda sorta full floating axle as Bryan says...because the weight is NOT carried by the axle...but by the hub bearing that is connected to the lower arm...just like any front suspension.
Precision because of how IRS works vs a live axle (solid tube and rigid one piece from left tire to right tire).
The weight from the frame rail is carried by the coil spring down to the swing arm (A arm, or lower arm) center (approx).
Then it swings on the pivot at the diff, a bit more than half way (guessing from the image)...to then place the weight to the pavement through the tire
Those pivots at the diff are NOT square laterally to the vehicle, but at an oblique because that is to manage the forces of moving the vehicle
Bottom line, for me, is that IRS is NOT stout enough for towing heavy and if over the spec/limits/ratings....going to wear out those bushings in a hurry
