Just google Tacoma weak tailgate for plenty of links. The gate is steel, but thin/weak. I had a 94 Nissan and a 94 Toyota and never had problems with those tailgates when transporting bikes. But the 2nd generation Tacoma gate is notorious for weakness.
To make matters worse, they used cables rather than brackets as the gate supports. The cables stretch under pressure, causing the gate to droop when transporting a heavy load. The cables do recover after the load is removed.
It's bad enough that there are commercial products to reinforce the gate, for example http://www.lowrangeoffroad.com/2005-2015-toyota-tacoma-tailgate-reinforcement-skin-with-ribs-by-mobtown-off-road-4p9va-05.html. That link has a good picture of the cables.
