nevermind nevermind!!! damn... i miscalled this one....
OK. xlockmore wants libGLU.so, not anything from the nvidia package, and that's where the problem was. At the point when I built XFree86, I had GCC 2.95.3 installed because i needed the old libs to build a new java SDK (it requires an old SDK to already be present... chicken and egg problem)... and apparently, somehow XFree's libGLU.so got linked with the old C++ libraries. I have a hard time understanding how, but that's what happened. So now I have rebuilt Xfree86 with no old libs in sight, and problem solved.
Thanks for the help bwkaz, the thing about the libGLcore error set me thinking in the right direction.