I lowered my 90 Formula with Eibach Sportlines and changed to Tokico shocks and struts almost 3 years ago. The ride was as expected: stiff but comparable to the WS6 setup. Over the past 6 months the rear has developed the feeling that it is bottoming out or quite possibly riding on the bump stops. I tried different settings on the rear shocks but it was either too stiff or too soft. I even changed the rear shocks to nonadjustable units and for a while the problem seemed to pass but has returned and seems worse especially on rough roads. The suspension setup goes like this:
Eibach Sportline springs
Tokico Illumina adjustable struts
Gabriel Gas Ryder shocks(in place of Tokicos in rear)
Lakewood LCAs and adjustable panhard bar
Spohn LCA brackets and adj.torque arm
Polyurethane bushings and end links
I also have Southside subframe connectors and an Edelbrock strut tower brace and IROC wonderbar if that makes a difference.
I still have the Tokicos fromthe rear and have considered putting them back in but I'm wondering if something else could be wrong. I've also seriously considered replacing the Sportlines with a ProKit to see if that helps. I've made a visual inspection of the entire rear suspension and nothing appears to be out of place in any way. Can anyone help?

 

Take a look at the rear bumpstops, are they contacting the Axle ?

They will leave a Clean shiny spot on the pad if they are.

Its possible that the rear spring isolators have worn out and gave up, especially if you re-used the stock Ones When you Lowerd it.

I don't have the PN handy, but getting new ones from GM did wonders for My Ride. Also lifted the rear about 1/2" above what the worn out stockers did. Helped keep the rear off the Bumpstops, But mine is severly lowerd.