Hey,
Unfortunately if the original injury did cause an injury to the PCL (or other ligament) you are likely to have some residual laxity in that ligament, which can't be changed and therefore on testing will always show up as some instability for that knee. However, these tests are designed to isolate those structures without the dynamic stabilisers being involved i.e the muscles around the knee, so when you take into consideration the overall stability of your knee including structural (ligaments, capsule, Wikipedia reference-linkmeniscus etc) and dynamic stabilisers, it sounds like your knee is already very stable taking in to consideration the activities you are and have been undertaking without any symptoms!
So it's just a case of maintaining the dynamic stabilisers as best you can, so I agree with the other post about strengthening the gluts, h/s, quads and calves. I would suggest also looking a lot at your core stability such as pilates exercises as these play an important role in lower limb stability. Strengthening should concentrate on concentric and eccentric strength and shouldnt just be using gym equipment as this doesnt reflect dynamic activities, so lots of plyometric exercsies aswell would be good!
Hope this gives you some ideas, any questions feel free to pm me!
Glenn