4kq diff lock servos

[walter@quattro.corp.inmet.com (Walter Meares)]

I have an 85 4kq.  It looks like both diff lock servos have failed.  The
rear one has been weak for a couple of years, and a few days ago the
center one failed altogether -- the rod won't move at all when vacuum
is applied to the "engage" side (although the pull-off side stillworks
fine).

Any suggestions on sources and costs for these things?  How hard are
they to replace (the rear looks accessible, the center one less so)?
Is it possible something else is wrong that I've overlooked?

Thanks all,

Walter
85 4000s quattro