Got a '92 w/45K. Runs great and handles great with the glass top on. But, take off the top and it shimmies and shakes like it's gonna fall apart. Any suggestions on how to tighten it up?

With the top out the only thing your going to be able to do is buy a brace that will help some. There isn't much to be done in this department except keep the top in.

has your car ever been in an accident, if so it may not have been reapired correctly, you can but a brace they make for the vert's but its a pain to remove the driveline assembly then[:@]
try these:(hope it helps)

http://www.livermoreperformance.com/camaro_vette_chassis.html
http://www.bmrfabrication.com/F3-chassis.htm
http://www.thunderracing.com/catalog/?action=categories&pcid=60

Unfortunately,the C4 was never known for being the most rigid car in the world,especially with the top off.There is this brace Mid America sells called the Targa Truss Bar.It gets bolted to the roof just like the factory top.It makes the car look like it has T-tops while at the same time strengthens the car and stops the shimmying and rattles....but it's pricey....about $400.But it looks like it works.I'm thinking of getting one myself.

Here's the reason, and the solution:

http://www.vetteweb.com/tech/0209vet_c4_coupe_anti_flex_support_bar_install/index.html

They had a last-minute design change. T-top ---> targa top.

Convertibles don't shake the same way our targa's do...[:@]