I've had the Extreme Trimmer rack 3 position/ Series one (gray/2006) and Series two(yellow/pictured) and had one failure to the front post (non locking) of the Series One model in which the shaft mounts were installed correctly. The design difference between them was the series one had a flat plate welded to the bottom of the post in the shape of a upside down "T" and the series two has a wrap around gusset at the bottom.
Both were mounted to a heavy duty 2" square tubing open trailer which will not flex and twist like the angle iron. However, the problem is still the weight and movement of the trimmers in the rack. Now the trimmers don't rotate or move within the rack but the rack still sways and bounce/flex on the trailer...
That was, until got the Engine Support Kit!
Before I did, I had already broke the shaft on my edger from the bouncing/shaft flexing on the rack. Since the installation of the Engine Support Kit, nothing sways, bounces or moves, no matter how rough the road. It eliminated all of these issues!! I highly recommend it.