It happens and I've been there. Repair isn't bad at all. First, drain the fluid from the front differential. Stand the Grizz up on the back rack. Remove the front right tire and the lower a-arm from the frame side. The differential is held in place by a SPRING on the engine side. Tie off the front differential to the front bumper, I used a ratchet strap since you'll need to pull the differential towards the front of the machine. Remove the front differential bolts and pull the differential forward towards the bumper, several clicks on the ratchet strap should get you there and the drive shaft will "drop" out of the drive couplings.
Replacement parts, two oil seals, (differential and engine side), two drive couplings, (differential and engine side), front driveshaft. Optional is the spring.
Parts from Parts Discounter on ebay should get you going much less expensive than OEM. OEM, hit up partsharkdotcom.
Good luck and let us know if you run into any problems.
Cheers

 

Good info from Hammerhead, this may help too. Scroll down to the bottom of this link and there is a service manual download for 02-08 660. You'll find the procedure in there.

Troubleshooting | 501 Parts.com Yamaha 4wd System Repair Parts & Service

 

hammerhead ?

those arent to bad to tear down other than the coupling was stuck to the gear in diff. but i managed to get it apart w/o breaking anything. im sure everyone here uses a different type oil in differentials what do you use 80/90 wt gear oil ? any brand ? normally use oreilly house brand seems to do the trick. ordered a new bearing for the gear the coupling mates with inside diff. am i going to have to open the case to gain access to that bearing? called (drive 2 pinion) fits on the nose of d.p. (#5) in parts exploded view. (bearing #28). havent got to look @ that yet. hoping for input prior to exploring. dont want open case if i dont have to. looks like a needle bearing if so i can get it out . maybe i should open the case just to look around for wear and the like. got it used few yrs back. the oil came out kinda brownish almost tan . what do you think ? wasnt leaking on outside of the case. thanks for your help.

 

Good to see you were able to get this removed. I did not have to replace any bearings when I replaced the drive shaft so hopefully oleblue or someone with experience here will chime in on the bearing replacement.
As for hydraulic fluid, I run amsoil marine grade 75/80 weight, blue cap. Capacity is a little over 7 ounces so you can get several changes out of a quart.
The murky fluid indicates that moisture more than likely got into the differential through a bad seal either at the axles or the driveshaft seal. Again oleblue or others may have better information on this and if you should flush it once you get it back together.