Thanks for the reply!
I removed the heat/sound barriers yesterday afternoon after reading that you used a heat gun to do it. It didn't take me long. It's sticky stuff and it feels a lot like tar or roof shingles when it's warmed up and ready to be taken off. Very similar stickiness and consistency.
I didn't have a heat gun lying around so I had to get creative. At first I thought of using my girlfriend's blow dryer to heat it up. That probably would have worked, but I couldn't find it.
Then I figured that maybe the sun alone was strong enough to warm it up. Turns out it was. I took the soft top off and let the rear barrier (underneath the rear seats which were taken out,) bake in the sun for a little while. I was then able to use a flathead screwdriver and a hammer and lightly chissle it off. It came up quite easily in large patches. There are light scratches on the steel of the tub from the chiselling, but it's not a problem as it will all be sanded down eventually and will be covered in liner.
For the barriers on the floor of the driver and passenger side, I had to try something different. I managed to get about half of each one up a la sun power as previously mentioned, but the other half was hidden in the shade. I turned on the heat full blast and let the Jeep run for about 10 minutes. Worked like a charm, and the remainder of both barriers peeled off just as easily as the stuff in the rear.
Today I also filled in the holes behind the driver and passenger seats with foam. There was overflow on one of the holes as I ended up putting wayyy too much in there, but I cut it off and sanded down the surrounding area. Looks decent. Soon I am going to see about welding steel plates over those holes. Unfortunately welding is not a skill that I have, so I'll have to take it to a shop to get it done.
I think I'm going to keep updating this thread as I continue to work/prep the Jeep for the liner, for both my own amusement and to help out anyone that may have similar questions in the future. I'll also try to include some pictures with the work that I'm doing or have already done.
I'm still interested in replies from anyone else that's done this. Lots of questions to be answered still. Thanks for your time and good luck