you could check the shifter rigging. see if the cable has stretched.
I4 & V6,
http://www.autozone.com/autozone/rep...1cce6c#hd1-1-2
another way is to, engine running/e-brake applied, depress the shifter button, hold it depressed, slowly move it out of park and see if she engages in between selections. if reverse doesn't engage while doing this, and she can still go into drive, likely internal trans issue. only checking the rigging will be definitive though.
you can also disconnect the cable at the trans and by hand shift it. i'd have someone sit on the brakes while you do that.
if that checks good, it's trans internal issue. you might change trans fluid and filter. but i doubt that would make a difference.