I have worn out a lot of the cartilage in my left hip such that I cannot run anymore and walking can be a bit iffy at times, and, for many years, I could not cycle. I am not familiar with your condition and I have no idea whether my situation is similar enough to yours. However, I am now cycling without pain for the most part since October, 2007, when I decided to make a slow careful go of it (and after years of work with a muscular therapist who helped me get out of state of constant pain -- not kidding, couldn't sit, sleep, etc without some element of pain). I did have to very carefully work through some pain (i.e., when I felt pain, I pulled back, and then would advance again carefully) and I can now go 60-70 miles at 18-20mph average speed.
Along the way as I've progressed, I did not feel pain sitting. However, I could feel pain when standing and pushing hard during a steep part of a climb. And when I felt this pain, I would sit, get into a cog with more teeth and push back in the saddle and push hard -- when doing this I did not feel pain. Although, I now feel no pain when standing on these very same climbs; of course, I am in better condition, but, I also believe my form is better when stressing the muscles with greater effort.
I can't say what your experience will be and whether my experience is material to yours. But, you might think about testing stuff out and being careful. You might also want to talk to a health professional; also, look up the surfacehippy group on yahoo groups; you may find people with similar background there.
As an aside, I find that standing during a climb tends to tire my legs more in the long run. I stand when I want a burst of high speed. For example, there is a short steep climb on one of my regular routes that can drive me nuts on occasion; on some rides, I will purposely stand and blast to get over it more quickly (i.e., get it done and out of the way).
Good luck!!!