Quote:

I was getting into the yoga last year but noticed when my legs were stretched out my knee felt unstable.

The human body is a remarkable " machine" ;)
It's able to compensate for muscle weakness and some injuries.
Before your surgery, you had a bad knee that lacked stability. Your body responds by tightening your hamstrings to help limit your range of motion, in effect helping stabilize your weak knee.
The Yoga by stretching out your hamstrings didn't cause instability, it just exposed the weakness.
If your knee is fixed, Yoga can only help if done correctly. Their are a myriad of home yoga tapes that target specific problem areas. I know theres a number for lower back issues.
Nothing the matter with home exercise tapes, but I would recommend a few lessons with a certified yoga instructor to make sure your doing things properly.
Proper form is the key to any exercise you do.
Sometimes the home DVDS are lax in emphasizing that or the camera angles/edit doesn't give you a proper view of what you should look like in a given pose.
Doing your workout with a couple of mirrors for front and side views is a big plus for making sure your body is aligned properly ;)