The rear of the Tacoma frame is subject to fatigue and failure if excessive weight is mounted to the aft C-channel. For example a heavy bumper, fuel tank, and spare tire mounted to a swing-out tire carrier. All of those items are scheduled for my vehicle, with the rear bumper, high lift jack, and spare tire weighing nearly 250lbs. The rearward frame section, just before the shock mount is not boxed as the front 2/3 of the frame is. 2000 and older Tacoma's are even more susceptible to frame failure as the frame does not have the additional reinforcement at the first bend (by the shock mount). The 2001 and newer frames are much stronger there, but the repeated flexing of the frame because of the rear bumper combination led me to want the frame reinforced for safety.
The reinforcement completed by Demello Off-road covers the major bend at the rear tire wheel well. They add a 1/4"x4 steel plate to the outside of the frame for a total of 33" of frame section.
This is a good article describing an actual failure of a 2000 Tacoma frame: Read Article from Tonkota.net