I used Noico sound/heat deadening in my LJ. its a closed cell foam sheet that I rolled on a little bit at a time. Also because the Jeep sees a lot of moisture I sprayed bed liner over it to seal it, and then reinstalled the carpet over everything.
It didn't work miracles but it definitely helped. I had a lot of heat coming through the transmission tunnel to the point that I could nearly boil water in the cup holders. With the hard top on I can hold a conversation on my cell phone with it mounted on the dash on speaker phone which is a significant improvement.
it was also very light so it added nearly no weight, which some of the other options that use butyl rubber do, though I don't know if the butyl rubber may work better. I covered as much surface area on the floors as I could to try and get the most out of it.
I got everything needed on Amazon for very low cost as well.