I'm looking to build a trailer that I can haul the wheeler on behind the truck then double to pull behind the wheeler. I only have to do 13 miles of highway speeds so not to worried about burning up the bearings.( I think, lol.)
Axles really aren't hard...some 2"x1/4" square (or round) tubing and a pair of spindles. Just leave the spindles sticking out of the tubing 1/4-1/2" so you can weld around them. Make sure to protect the threads and bearing surfaces from spatter. I use a finger off an old leather glove usually.
I like to drill a 1/2" hole in the tubing an inch or two in and plug weld the spindle as well for some added insurance. Spring center pads are simple. Just weld them on at the proper width for your frame members (I'd do this after the frame is built).