Once I put antlr.jar on the $CLASSPATH and everything works fine.
Yeah, that worked.
Everything compiles and the gears demo runs properly.
Very odd problem. Hard to figure out why antlr.jar won't work when it resides in jre/lib/ext. Doubly confusing, since the "class not found" error is for a jogl class, not an antlr class. Perhaps the antlr code is reading the CLASSPATH and trying to locate itself for some reason.
Thanks very much for your help.