hello my 96 ranger will not go into revese what could be the possible cause.

If auto transmission it could be that you need to replace some seals or plates. Meaning a repair kit.

my 2000 zx2 is having the same problem with reverse. its an auto. could it use the same treatment??

The easiest way to start with any transmission problem is to see if a fluid and filter change helps first. A dirty filter will cut fluid pressure and things will cease to work little by little. If filter and fluid level are there - then comes time for the hassle of tearing it apart for seals and plates - but at that point it may as well be remanufactured (have new bushings, gears, etc. put in too) so you don't end up going over it again any time soon unless it was a new transmission or had low miles on.

And don't forget to start by checking for fluid loss (proper level). A cracked transmission line don't warrant taking apart the whole unit. I've made a mountain out of a molehill enough times to keep the simple stuff in mind first.