I have a 2006 Pre-Runner - TRD - 2WD truck. I carry a few hundred pounds of stuff in the bed most of the time. I noticed that on more than a few occasions that the rear of the truck seem to bottom out. I ask my local dealer and he told that there had been a "campaign" a few years ago to correct this. Of course I was no longer covered. I asked about replacement springs and was told they would cost about a grand. I had purchased some air lift air bags for a F250 I owned and had good luck with them.
The Suspension Connection sells a set for Tacoma's #2407 at a cost of $260. They were a breeze to install and have worked great. No more bottoming out . Hope this helps.