I believe the lightest is the Westin Safari Light Bar. Not really a grill guard. From there you can get center bumper guards and and full grill guards that cover the face of the truck. All of them bolt on.
A couple members have the Outlander Sport made by Smittybilt, that is what I was going to get, I think it looks good but I can not find them for the Sport Trac anywhere now.
I have the Outlander Sport Guard from Smittybilt. I purchased it from Summit Racing, however in doing a search I can't find it on their site. I have seen them on E-Bay in black and chrome. The part number is SMT-213319-B.