well i like the design, but i would highly suggest trying a threaded axle with a pin that fits into a hole in the axle and truck. this would allow for the strength of threads, with out the possibility of backing out. also, make sure you make a recessed cup for your bushings!!!!
reminds me of this truck, and the axle with pin thing is on this truck also
http://www.landyachtz.com/D27.cfm?PID=257&place=3