For the sake of convenience, I'll only include shocks which are a direct-replacement for the stock shock. That is, shocks which will bolt in without modification and which will result in minimal change to the ride height of the tail.
Many Seca II owners have replaced their stock rear shock with one from Yamaha Vstar 650. There are two variants of this shock: The Vstar 650 classic version and the Vstar 650 custom version. The classic version is a bit longer than the XJ shock and will raise the tail slightly. The custom version is identical to the XJ shock and will not alter the seat height. Both versions feature a slightly stiffer spring rate and are readily available on EBay.
Here's the stock Seca II shock and the VStar 650 shock, side-by-side (the V-Star shock is on the right):
Some Seca II owners have recently begun replacing their rear shock with one from an FZ6R. This shock is noticeably stiffer than the stock XJ600 shock and raises the tail slightly. Since Yamaha is still making the FZ6R, you can usually find a near-new one for around $100 on EBay.
Rear Shock Replacement Options