The easy way to tell what you need is crawl under the truck and see what's been rubbing. I haven't measured the height difference between a stock 4th gen 4r and a GX. I can tell you that I have two buddies with 4r's. One is still running the stock skids. Last year in CO we ran several rougher trails including Half Moon Gulch and Wheeler Lake. He would occasionally drag the factory gas tank skid, and did damage his muffler on one rock.
This year he was running 2 inches more lift when we did Rose Garden Hill and Hell's revenge in Moab. Once again the factory gas tank skid got drug a few times. The thing I noticed taking the most beating however was his rear lower control arm brackets. Several times they jammed up on rocks and we just kept him moving forward until they drug over the obstacle.
The point is you can do a lot on the factory skids even though they look super flimsy. A lot depends on your driving ability and experience. For as cheap as they are I would definitely get the little sliders that bolt to the lower control arm brackets. I could care less if the bracket gets sanded down a bit. But having them slide over rocks would be a big benefit. After that I would say protect the major stuff like your TC. To me the forward skid, even though a bit flimsy, should still be able to take one good hit. It may buckle, but should dissipate enough energy to keep everything behind it safe.