The only things that do anything worth your time in the home use light based hair removal market are Tria, Silk’n, and No-No (also marketed as No!No!, No No, NO-NO, and so on.)
The thing about these however, is that your results are really only comparable to waxing or shaving. If you discontinue use of the products, your hair generally returns with full force and fury in 6 months to a year’s time, if not worse due to the constant irritation of the use of the machines over time.
If the amount of hairs on the back is not all that much, I could not see a good reason to do L.A.S.E.R. on it, but that is a personal opinion of mine, that is not shared by many other people (although it IS shared by many others ).
If you can recruit a partner to trade work between the two of you, purchase of a good serviceable professional unit would be less than a one time treatment over the entire area you are discussing, and would allow you and your buddy to totally finish the job of clearing each other out in the course of about 2 to 3 years. During the time you are working, you may even look finished as you continue to work at it, if you can put in enough work to clear out areas, and then work on keeping them clear. One can see hairs via magnification long before they are visibly noticeable to a person standing a conversational distance of 4 feet or more away.
Good Luck in what ever you try.