Regarding not being able to instantiate a class although it is in the classpath: this might point to a problem occurring in a static initializer or the constructor. Have you run it through a debugger to see if it get to those places, and whether it makes it through?
I am not 100% sure, but I assume the Problem is JRE 1.2.
Class javax.naming.ldap.InitialLdapContext is avaliable since 1.3, and the same is true for interface javax.naming.ldap.LdapContext. These types are probably needed if your com.sun.jndi.ldap.LdapCtxFactory tries to load.