Hi Noisy
Do you recall what numbers the shock had on it. From the sound of it, given the frame layout, it sounds like a YZ 250/400 D/E shock. The shocks are pretty ordinary but certainly respond to a freshen up and new bump stop rubber. If you do get it serviced I have found that getting the gas pressure up to the maximum seems to help.
As it happens, I've been trying to suss out a heavier spring for my YZ250E. You may have helped immensely with my problem.
Any extra 550 springs out there (even to borrow for testing?)
Regards
Peter B