Hello David and welcome to SD1 ownership. First off, well done on buying the SD1.
That's a very good price for such a rare car in good condition in the US. I'm more
than tempted to get a US version SD1 and get it shipped back, but finding one in
good condition then actually sorting out the shipping is something I've not quite
had the guts to do yet

Any pictures of your car?
Regarding the Nivomat self-levellers. It sounds like they are shot on your car, I'm
afraid. There appear to be two modes of failure with Nivomats. Either they go rock
hard, or the go soft and drop the back of the car. The failure within the Nivomat
seems to be the seals. I'm guessing that the symptoms of failure depend on which
seal fails first. There is a method of repair out on the web for Nivomats, but it's
extremely dangerous due to the high pressures of the oil inside - it's something that
could be done easily by a small engineering workshop in batches, but not something
that could/should be tried at home!
New Nivomats appear on eBay sometimes - I think there are a set on at present
for £150, they are the non-Vitesse type which give you the correct ride-height.
Alternatively, Rimmer Brothers have NOS Nivomats - there is question over what
ride-height these give (the Nivomat sets the ride-height, not the spring), they
could be correct for your car with standard ride-height.