Hey guys, hate bringing up a old thread but wanted to pass along some info...
The movement you are seeing is from the flexing of the frame. No matter what bumper you install, that frame is still going to be bouncing all over the place. One of the differences with the ARB is that you can easily see the hoops/frame. The same frame shake happens to the factory crash beam, but its hidden under the bumper skin.
Fact is the Toyota has had a known issue since the Prado 120 platform was introduced. MANY FJ Cruisers (and going back to the Prado's oversees) have had their inner fenders split from the metal's fatigue caused be the constant flexing across the entire front end. Go take a look at one of the FJ Cruiser forums and read some of their experiences. All of that happens with or without an aftermarket bumper. From some of the polls they made, it happens more to stock vehicles. The going theory is that aftermarket bumpers add some rigidity between the frames above what the factory bumper does.
Those connections comes to play here because the Tacoma, 4Runner and Tacoma all use basically the same front structure. The inner fender tearing isn't happening as much, but the movement between the frame horns is pretty much the same. Fun stuff!