I'm not sure what length and stroke the shocks are on the LTS - if its fairly standard length and stroke you'll have plenty to choose from (Fox, Rock Shox, Manitou etc).
If not, Risse custom make shocks to pretty much fit anything.
I upgraded the shock on my Giant to a Cane Creek AD-5 (air shock) I bought cheap on ebay and couldn't be happier with it. Its very light and easy to service if need be. Looking on the website, it seems they make a version of the AD-12 (similar shock - with external adjustability) that fits the LTS 2 & 3. Would be a good option if you're wanting to drop some weight and set it up for XC.
Having said all that, if the current one works well and you're wanting to keep the thing as original as possible, I'd just stick with what it came with (comforted by the knowledge a blown shock in the future won't render it a write off!).
1997 Giant ATX 980
2007 On-One Inbred