The title says it. Motor runs fine until 4 wheel drive is selected with both diff locks. Then it runs so badly spluttering and choking it couldn't pull the proverbial. I think it's a 2007 model. Thanks.

 

Check electrical connections. All the shifting for 4 wd is electrical correct? If it runs fine other wise it could be an electrical issue.

 

Does it have a rev limiter when in reverse? Some Yams do, and it may be that Yam thought it a good idea to make it cut the revs when the diff lock is in as well.

501 parts, know more abut Yam 4WD systems than Yamaha dealers, you could try them. Troubleshooting ? 501 Parts.com Yamaha 4wd System Parts and Repair

 

I had a Grizzly 660, it would limit power in reverse in diff-lock. I think there was a button to override it but you had to hold the button. It was a bit frustrating when you're in a tough spot and try to back out while in diff-lock and it would cut power. Seems like when you're in fully locked mode, it would be obvious you need all you can get and the worst time to cut power. I guess it was a safety thing but it just didn't make sense. I don't think it limited power going forward though, just reverse. You might also need to be in low range for the rev limiter to not kick in going forward fully locked.