A big "Thank you" to both Charles and Wojciech for their responses.

I did the ktrace and found the following message occured while reading
from the NIS group table:

	su: initgroups(root,0): Invalid argument

I had recently added two secondary groups to the NIS table which were
somewhat large entries -- approximately 2 dozen users listed in each. 
The NIS table already had already contained several dozen smaller group
entries.  Removing these two large groups from NIS 'fixed' the problem. 
After a bit of testing I've determined that I was probably bumping
against a size limitation within v1.5.1 & v1.5.2 in regards to the group
table.  I've created a work around (for the group entries) for the time
being and will schedule these three systems to be upgraded.

I have regained root access to these systems.  Thank you, again, for the
help.
-- 
  /\---/\  Eric J Fox
 /  o o  \ http://fox.phoenix.az.us
 \.\   /./ ---------------------------
    \@/    "Of course it runs NetBSD."
.
Charles Blundell wrote:
> 
> on Fri, Nov 22, 2002 at 12:10:35PM -0700, Eric Fox wrote:
> >   bsd005(foxe): su -
> >   Password:
> >   su: setting user context: Invalid argument
> >   bsd005(foxe):
> 
> setusercontext(), the function that raises this error, logs more
> verbose information via syslog. you may want to log in as root and
> have a look at /var/log/authlog for more details.
> 
> if this fails to reveal anything, whilst as root (in single user mode,
> i suppose) you may want to try running ktrace on su, and seeing what
> fails last by examining the output with kdump.