Rear end feels 'floaty'

[Fuzzy]

Sounds like it might be past it's best -  could be worth getting it rebuilt. There are places that will rebuild the oem shock despite it being officially non-rebuildable. Usually costs about £100. A better alternative is to buy an aftermarket shock but the downside to this is the cost a decent shock will set you back £300+ unless you get a Hagon which is slightly less (I wouldn't personally).

I think I'll start a thread with information about shock options including rebuilds so keep an eye out in the next couple of days.