All the shock absorbers have some adjustment for ride height but it's really only the aftermarket coilovers like KW variant 3 and Bilstein PSS9 that have additional adjustment for bump and rebound of the dampers. If you've got the standard Boge shocks or Bilstein HDs (which are a popular replacement for the Boge shocks) there's no adjustment to their damping action (adjusting the ride height won't have any effect on the damping capability of the shocks). Therefore if your car feels like it's wallowing it's likely that your shocks have had their day and need replacing. The concensus of opinion seems to be that if the car is on original shocks, that after nearly 20 years they are bound to need replacing.
With normal cars it was possible to tell if the shocks were bad by forcing the wing down with your hands and letting go. If it settled straight away they were OK but if the wing oscillated a couple of cycles they were shot. However suspension on a Porsche is so stiff in comparison this test is of little use.
The other thing to consider is that if you think the rear of the car feels soft and has a tendency to bounce up and down after going over a bump, it could be that your engine mounts are worn out. Changing these can have a dramatic effect on the stability of the rear of the car.
