That ladder truck would be cool converted to rc.
i use the axles from a mini giant rc monster truck but, they quit making them many years ago and rarely come up for sale on ebay.
But, if you only need rear wheel drive you might check out the tiny metal axle that rc4wd sells. It is a 1/18 t-rex micro axle. The axle is very small but it is all metal so, you could extend it to the width you need..
http://store.rc4wd.com/118-T-Rex-Mic...xle_p_438.html
Good luck and have fun.