Cam,
Remember you're driving a 'high profile' vehicle with large flat sides,
lots of sail area.
Starting with my dearly missed '71 Westy, I've put a rear anti-sway bar on
all my vans. The last I bought for my '87 GL from Addco Industries in Fla.
Their number is 
800 3387015. I believe they have  web site now tho I don't know th exact
URL. Seems I paid about $170. I installed it myself in a couple of hours,
did require drilling some holes. It really minimizes the wind gusts and
turbulence from trucks. My wife was pissed off when I bought the first one,
she thought it was a silly waste of $ til she drove it once. I suppose
there are others that make sway bars as well, I've only dealt with Addco
and been satisfied. Now at 158k miles on my GL, I need to rebuild some
front suspension parts to take the looseness out of that area, but that's
another project...:-)
And yes you may need new shocks. I've had good service from KYB's.
Cheers!
Jess Lopatynski
89 911 Targa
87 GL
85 GTI