The only company that sells offroad skids to replace the existing ones found on the 4th gen 4runners are at Downey Offroad.
They are the exact same size as the stock two piece except it now becomes a single piece design. It weights approximately 40# or more but can take a beating. I had their skids for approximately 9 months before upgrading to a different design.
The current design I have is a prototype skid plate that covers everything that the stock covered including the Tranny (Downey's skid does not cover this piece).
If you are interested in the downey skid, please visit their website. If you are interested in the prototype skid that I am running, please be patient as the developer is still working out the details. I will be posting that information once it becomes available.
Good luck