Sounds like the shock is defective, which should be replaced under warranty. Has it had a hard life? Mileage doesn't mean much unless it's only been on paved surfaces and assuming you're not a sumo wrestler.

As for the 800GS, I'd rather have an xt with ohlins and the extra cash in the bank. Doubt it will stand up to winter any better and the dealer won't thank you for doing any home wiring either.

Hyperpro can do a new rear spring for about �80-�100 and progressive fork springs too for about the same. Might be worth considering, if you don't want to shell out for a Magura or ohlins replacement and don't get any joy with the dealer.