Could be arthritis setting in. I hate to say it. Also, I find treadmills don't do anything for me as far as trail conditioning is concerned. The starimaster and elliptical (on high resistance) seem to work better, but again, you have to watch your mechanics.
Going backwards on the elliptical with relatively flat incline and high resistance will help with training for downhills, but nothing really mimics hiking like hiking. Your center of gravity is changed, the terrain is uneven and different, so your footstrike and gait will vary.
I recently bought an open-patella knee brace since one of mine tends to swell up like a cantaloupe. I take Osteo BiFlex when I can remember and try to lay off the ibuprofen- that stuff is bad mugambo for your liver, and mine gets enough work as it is.