I'm pretty sure the quality of the UMI's are better. BMR is another place that makes trailing arms for the w-body. Don't get the one's from wbody store. I have read that they are dorman stock replacement railing arms. You can also wait and see if someone sells thier GMPP one's in the for sale section.
Here is the part number for the GXP trailing arms. I have read that they do fit and are tubular.
Trailing arm, grand prix, w/special suspension 2005 - 2007 $27.78EA
PART# 15856683