I had a two inch sheet metal welded to my GLH's body rear subframe's U channel. At first, the rear suspension was really stiff, it would lift the rear wheel when there is a slight incline on the parking lot I drive into. Then, I felt the rear suspension get softer and softer as it no longer lift the rear wheel as much anymore. Maybe a year later since I welded the subframe, I noticed the cracks.

I now uses a n/a omni subframe with a polybushing sway bar.
http://polybushings.com/pages/swaybarLrr.html

My advise don't box up the U channel or you'll be needing to replace it soon.