You need to get the hot air out of the engine room. There's not enough space under the lowered truck for the air to escape there any more.A quick way to verify this is to run it for a while with the hood off, it'll cool right down. Sometimes the hot air recirculates, too, is there a sealing flap between the rad support and the hood?
I've seen guys remove the fender liners to let the air out there, this may not be practical in your case, but it does work.