<<   yes there is. if you can find a junked synchro,
you can scavenge the  differential parts from it, and
put them in yours. maybe.  >>

(about installing some sort of locking type diff in a
091 style gearbox.)

I agree but it would require a complete teardown and
replacement of the main transaxle housing to one
for the locking diff type.  Big job, big money, and big
headache if you are trying to do this yourself.  What
you might consider instead is the 'limited slip' type
add-on part from Weedle Engr. in California.  All you
have to do with those is remove the differential from
your trans, install the parts, and then reinstall the
diff.  Not nearly the big headache and cost of retro-
fitting a locking VW diff into a Van which orginally
didn't come with it.  Adding the Weedle Engr. kit
should also be within the technical skills of a vast
majority of most home mechanics and does not
require a large array of speciality tools.

Todd Hill
VolksWerks Transaxles
Olympia, WA