Problem Upshifting

I'm not sure if it's because I have a 16yr old transmission. But for quite awhile now it seems that the car isn't getting into gear properly when slowing down at a stop then speeding back up, there is a very harsh engagment when upshifting from a slow speed. As soon as i step back down on the peddle it clunks when it finally manages to get back into gear. Sometimes it feels like im getting hit from the rear.
Any solutions??? other than the dreaded rebuild.
Thanks in advance