Hi Thomas – I recently ran into a similar sensation, especially when I would extend the pained leg for a reach or a very high step. I asked some peers and they suggested it might actually be a strength issue, in which case I should do fire-hydrant excercises before climbing and on off days to strengthen the hip flexor. After two weeks of doing these excercises 10-20 times on both sides before climbing and on off days, I noticed the pain has subsided and I have a full range of motion. Hope this helps!
LB