So I'm gona swap the rear shock out on my 1990gs.  I was going to use a 89-95 gsxr but I can't seem to find any as of late.  I was reading up on the kat shock switch...one site said that its for a 95+ though.  Is this true? 

try this

http://cgi.stanford.edu/~sanjayd/gs500/Upgrades/RearShock

http://pantablo500.tripod.com/id12.html

http://pantablo500.tripod.com/id12.html

95+ what?
the late model Kat shocks bolts right in to the 90+ gs500's, so long you get the Kat shock that does not have a resevoir (I think thats the kat 750, not sure). be careful on ebay as often shocks are listed not by what bike they came off but what they'll fit-for example (and I'm guessing as to fitment) a RF900 shock might be listed as a gsxr shock if it fit the gixxer...you have to ask the seller specifically where it came from. the Kat shock will look identical to the stock GS shock. get the newest Kat shock you can that looks identical. I had gotten a 2001 Kat shock for mine, but ended up getting a good deal on the gsxr shock (which is impossible to find, and usually in worse shape since they're older).