What I do on my truck (solid axle, but basically the same as far as toe in goes) is to just sight down both front tires and see where they line up on the rear wheels:

http://www.4crawler.com/4x4/CheapTricks/AxleTech/index.shtml#Alignment

Since my front and rear track widths are different, I can't use strings from front to back (for which I assume an equal track width is required). I just use a consistent mark on the front tires to sight down, like a line of tread blocks or a groove in the tread and see where that line of sight hits the rear tire tread. You can get pretty darn close that way and not even use any measuring tools, strings or anything.