[White Zorro]

Recently, I noticed a problem while reversing, it doesn't engaged. In several occasion, it is okay but then again, not engaging properly. What could be causing this problem? I have a 3.1L V6 TransSport.
Please let me know if it is really that serious, I'm worried about it, problem may become more if left unattended...
I appreciate those who will reply to help me solve this problem.

Regards,
White Zorro