Saber6 Is correct
I did one of these the other day. If your TRX680 is crunching into FWD or RWD or simply one of these, the dogs are worn. It doesn't take much as shown in the photos above. This is a very common problem as the trx680 differs from the other trx models because it uses a torque converter (which honda call a hydrostatic clutch) like a car transmission rather than the too well known and indestructible honda steel centrifugal clutches. Honda did this because the trx680 produces to much torque which destroys the normal steel clutches that the other lesser powered models use. This problem seems to occur every 2-3 years with these bikes.
First drain oil. Flip the bike onto its front bullbars, (this way all the gears and shafts sit in place due to the gravity). Rip the entire rear end (rear axle and drive shaft) off to allow full access to the rear of the engine. Unbolt the pull starter and the bolts inside and disconnect the wiring and shifter cable. Lift the cover off and replace 1 or both of the dogs. Dogs cost Approx $180NZD for a pair as of Dec 2011 and are available through any honda atv dealer. Replace dogs. Seal case with permatex or similar (threebond is good) and ensure no sealant has got into any galleries or will get into galleries once cover is placed. Alternatively purchase new gaskets. Bolt everything back together. Replace oil and drive away.