Hi Dave,
Here is the Rockwell website:
http://www.rockwellamerican.com/productsAt a quick glance, it looks like their torsion axles are similar to Dexter in that you can order different weight capacities & start angles, ect.
Be careful to match the axle capacity to your loaded trailer weight (i.e. a 1000# trailer with a 2000# axle will ride very rough). And also be aware that the start angle determines your ride height.
In addition, half axles require different frame mounting & alignment considerations than a beam type torsion axle.
Two threads to study:
viewtopic.php?f=35&t=14125 &
viewtopic.php?f=35&t=7214Good luck with your build & be sure to post pics.
Fred