They replace the rubber bush under the front end of the diff, that connects the diff to the front cross member. When larging it, the diff nose is lifted away from the cross member by the engine torque, which after a while buggers up the rubber bush, hence the stupid strap thing you'll also find going over the top of the diff nose.
So, solution 1, replace rubber with metal and everyone's happy? Well apart from additional noise and vibration from the diff being passed through to the chassis, and slightly increased risk of chassis being tweaked by large torque loads not getting absorbed by rubber. It's definitely one of them compromise things.
Cheers,
Rob