For anyone looking for this in the future: Congratulations, you're about to learn how to be flexible,
On my '14 FJ (Offroad package, if that counts at all), I was able to shimmy under the frame while parked in the driveway. Many will have an easier time than I did; I'm 6'5", 340#, and old/brittle. I found that there was only one area on the muffler heat shield that had given away, the one roughly centered at the rear. The nut/stud was in good shape (Krown treatments help), and I was able to use a 3/8 drive rachet with a 10mm deep socket to remove it. I then used a 1/4" x 1 1/4" OD Stainless Steel Fender Washer to hold the shield in place, and retorqued the nut into place.
The hardest part was figuring out how to reach. I had to go in left handed between the drive shaft and exhaust output pipe. While not HARD, so to speak, I think that someone a little more bendy/skinny than me would have had an easy time doing this.
All told, I was into and out of the garage in less than 20 minutes.