Requirements
1. bullet-proof log hauler that will pull logs or trailers of logs on the side of a mountain
2. EFI would be important for reliable starting/running at any altitude (but more stuff to go wrong)
3. Independent rear suspension would be best while driving on the side of a mountain.
4. Unlockable rear end for not sliding on side slopes - maybe like the Polaris Turf mode.
5. Low gearing and tons of torque for pulling.
6. Low center of gravity to not tip on side hill.
7. Low speed - hauling only - no sport riding.


I currently have a 2000 Grizzly 600 and am in the Colorado mountains at 9000ft.
The Grizzly has the following issues:
1) Carburetor - runs okay sometimes but mostly bad - has been rejetted and serviced several times.
2) Solid Rear end - slides when hauling logs or a trailer on the side of a mountain.


Thanks,
Steve

 

Lots of models to choose from out there. I don't think that an unlockable rear will help you very much - you are likely to slide on a side-slope anyway plus there are only a couple of unlockable rear ATV's out there (Polaris and a small Kawi). With pulling a heavy load in the woods, you will need more than one-wheel drive traction.

You probably need the traction of a real 4wd with a full time locker so a Yamaha or Suzuki on/off locker set-u will work better than the sensing types in Polaris or Can-Am or the progressive Kawi locker.

Overall I think that an EFI Yamaha Grizzly or Suzuki King Quad will work best - both also have low ranges.

 

A Sportsman Big Boss 6x6 800 or Sportsman X2 850 would be great for heavy work, especially the 6x6.

 

X2 on these two machines. The 6x6 Big Boss has 4wd that can be switched to 6wd, 800 lb. load capacity with 1500 lbs. of towing. Nice storage box under the front rack for straps, and other gear. Don't think it has the Spicer rear diff. with Turf Mode. 2012 Polaris Sportsman Big Boss 6x6 800 ATV : Overview is the website with information. Here's a couple great videos showing the abilities of the 6x6 and X2.



The X2 in 550 or 850 most closely matches your criteria. Has the Turf Mode for turning in tight spots, EFI, 1225 lb. towing, 400 lb. rated rear dumpbox/seat that can convert between a dumpbox and a passenger seat in 10 seconds, good under rack storage in the front, good in low range, and has good engine braking with the ADC/EBS combination. Polaris quads are 48" wide which makes them very stable work platforms. Both the 6x6 and the X2 have a longer wheelbase than regular quads so they are more stable front to back than a regular quad. No one makes a more utility oriented quad compared to these two.