It's a foam piece. I just replaced my 6 spd.

Thanks! Does it go underneath the inner shift boot? In between the boot and the transmission?

It sits on top of the transmission. It is underneath the inner shift book. It sandwich's between the transmission and the tunnel.
You may need to take the shifter off and drop the trans slightly to get it on. Or since its foamy, you might be able to work it in place from inside.

I would try inside first. Be careful you don't tear it.

So after removing the outer boot, inner boot and shifter I was able to get that foam donut in place. I do not recommend trying this, its a round peg in a square hole and really tore the donut up. However, its in there very tight and it seems its has created a tight seal. I was able to get the new inner shift boot in place though it was a tight fit with the foam donut not being pushed all the way through. Also the shift knob broke in the removal process so I got a new one coming.

For anyone following this, changing the inner shift boot and the foam donut seal has tremendously lowered in can temperatures as well as noise. Its so much quieter and so much cooler inside!

another solution
 

I have a 2008 JKU with LS3 and 6L80 transmission. The car gets very hot in the summer and the added heat generated by the engine makes the a/c work very hard to cool the vehicle in the summer. Hot Heads Headliners sold me panels for my roof as well as their sound absorption product called Sound Assassin for the floor. My goal was to reduce the reverberation in the car caused by the larger engine and to reduce the load on the a/c system in the summer. I have a black top and the a/c really has to work in 90 plus weather as well as try to reduce the added heat from the V-8 and full cat's. After installing the sheets of sound absorption material which have a foil backing over the entire floor of the vehicle the car was not only much quieter but the heat gain from under the car was reduced significantly. I can't say how much the roof panels will reduce the heat gain from the roof until this summer but so far the $600 I spent was well worth it. I don't go rock crawling with the car but I can imagine how hot the car would get in the summer doing so. I highly recommend the Sound Assassin treatment to anyone trying to reduce inside noise as well as heat gain from under the vehicle. It goes without saying that JK's have numerous floor plugs and penetration through the fire wall and these must be closed off to keep heat gain from under the car and from the engine compartment to a minimum. Hope this helps a little. There are similar treatments by other companies too but this is the only one I have experience with. Their link is below:
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