There are numerous walk behind electric fork lifts available, but as far as I am aware all are designed for smooth floor service.
http://www.liftscience.com/Is this is along the lines of what you are describing ? I don’t know of any framers using one.
http://www.bobcat.com/loaders/product_lines/mini_track_loaderI’m not sure they can be had for much less than the cost of a normal skid steer, and they are not going to get trucks loaded or unloaded for you. Here’s a current ebay listing
http://cgi.ebay.com/2003-BOBCAT-MT50-WAL...1QQcmdZViewItemA quick web-search will show you that Toro makes something similar in the happy homeowner class, but it won’t share the convenience of the quik-attach forks…
I’m still what I call a low tech framer, but feel hydraulic material handling is a required investment that both saves you and your clients money, and preserves your health.