Here is a link:
http://www.purplelineusa.com/products-enduro-about.php
I got it on sale for $600 plus $100 S&H.
The movers that attach to the hitch do a pretty good job, but their tires are small & only 10-15% of the trailer weight is transmitted down the hitch to that type of mover. So in grass, dirt, sand, etc. they would be very susceptible to loosing traction & spinning.
The Enduro mover turns the trailer wheels so there is very little chance of loosing traction. However, it is very important to install the roller of the Enduro mover EXACTLY 20 mm from the tire so that when the roller is engaged against the tire there is enough force to prevent the roller from slipping. The company ships these little 20 mm spacer blocks to use about positioning the Enduro motors. There is a cross bar that runs across from one motor to the other so that when you engage the motor on one side, the other side gets automatically engaged.