A good start is to check the fluid level and see that you are getting circulation when it is running. Then, get under the car while it is running and move the link to the differential up and down, to see if the suspension reacts. Could be you only need the accumulators. These are a regular maintenance item on these cars, and will cause a hard ride as the rear suspension is solid without them. I posted something about replacing mine a long while ago. It is not a huge task, and they cost about $75 USD each side. The fluid is a bit pricey, too, but I can't see where you will spend $1480 as you indicate. That sounds like a lot of money to change over to a lesser system. Mine rides GREAT with the pneumatic rear suspension, I would not trade it for anything.

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1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)