This is only my second pregnancy to have anything to compare to, and so many things feel different this time. I honestly can't remember feeling a "lightening" or engagement last time, my husband thinks it was 5-6 days before the birth but I have no idea! I read it happens closer to birth with later babies, but surely there are degrees of dropping, and babies move... I DO remember having awful nerve pain in my hips and thighs but that was in labor, when the baby was much lower! And fortunately I had been with a woman in labor before who had the same thing happen, and I knew what was going on and that it was going to be relieved as soon as the baby shifted and continued on down.
I did want to encourage you with the UC intention - that is, to encourage you to listen carefully to your instincts and maternal knowledge of this little baby and when he/she is ready to come into the world. I UC too, and remember those weeks well - having a few too many contractions early in the third trimester and wondering at what point it would seem okay to have the baby at home still as planned. 34 weeks? Probably not. 35? Dunno. 36? Uhm.... maybe? I think it was around 36-37 that I felt a definite sense of the baby being big enough, strong enough, that if everything just felt right I would stay right here and go ahead and have the baby. (At 38 now, there's no doubt, though of course it's probably best baby stay put another week at least!)
Good luck, and I hope you are able to relieve the nerve pain in the mean time... the yoga tricks I know for sciatica all involve lying on one's back and probably wouldn't be so good with a big belly. I have had good luck relieve back pain this time with swimming/just floating in warm water.