I guess if you do not have the spring assist Nivomats, the non-self leveling IRS includes on of the two heavier transverse springs.

I see only one of three available here (we're interested in the part #s):

Swedish Autoparts: VOLVO S90 SUSPENSION REAR 1997-1998
http://www.swedishautoparts.com/S90/Volvo-S90-suspension-rear.html

If you can find the part number on it, maybe you can find another transverse leaf spring of heavier rating?

Else, if no Nivomat, yet with a multi-link rear suspension, and the press indicates you do, all dampers I find are gas-charged with no coil over.

Also, if only found at the Volvo stealership, try Tasca Volvo parts. The various Volvo parts outlets show all leaf springs as NLA.

Avoid Made in Mexico Sachs. I'm unsure even Volvo OEM.

Also, for the number of miles, consider rear suspension bushing inspection and replacement.

As an aside ... if you are unsure when the two timing belts were replaced, consider replacement of them. The white block B6304 is an interference engine. Also, proper coolant replacement (distilled water or premix), and so on.

There exists a Volvo TSB (tech service bulletin) on timing belt replacement interval. Though I'm unsure how frequently.

https://www.brickboard.com/FAQ/700-900/EngineSealsBeltsVent.htm#960TimingBeltChange
--
Give your brickboard.com a big thumbs up! Way up! - Roger Ebert.