I hear of rear sway bars being avaliable. A simple bolt on , I believe.

My late 85 has the solid cast iorn lower control arm that all the square
headlight  vanagons have. This 85 retains the thicker sway bar that the
sheet metal(early) lower control arm vanagons had. The thicker bar is 21mm
in diameter and the thinner bar is 19mm in dia.

A swap out for this thicker bar is ez. Besides the bar(free?),the 4
bushings need changing.That is all. Takes about 1 hour.

I dont know why VW switched bars,but maybe a thicker  a bar is what you
need.