I've got a 2003 Yamaha Grizzly 660. A couple of months back, I had two CV boots (1 rear inboard, 1 front inboard) tear open. This machine sees mostly just farm use and didn't get into anything that could have torn the boot open. I chalked it up to an original boot that might have gotten old and cracked. I picked up some replacement boots off of eBay and replaced them, but now I noticed the front one I replaced is torn open again. This machine hasn't been through anything that could have torn the boot since the replacement, so I'm at a loss for why it failed.

The machine does have a lift kit on it, but I've got buddies with similar machines that don't have any boot problems. What else should I be looking at? Or does anyone make a HD replacement boot meant for lifted machines?

 

How big of a lift kit does it have? What are the angles like on the axle shafts maybe that has something to do with.

 

If you are running stock axles, then that's your problem. Because you can't lift a 660 Grizzly without running into CV axle problems without switching to aftermarket axles. I have never heard of any manufacturer claiming to offer a kit that will work for stock axles due to this reason.
Yamaha already has them at a stepp angle right out of the box,..which causes CV boot problems. Owners who lift them with stock axles run into issues such as excessive CV joint wear, ripped boots, and rear axles popping out of the rear differential. This has been discussed on this website and others for several years.
You have some guys that somehow never run into issues, and then you have the guys who do have issues. It sounds like you fall into that second group.
My advice to you is the same as what I use for the locals who have the same issue you have,...go to a dealer near you who has a selction of different brand boots and pick out the thinnest and most pliable boot possible. It's usually the cheapest as well.
Although that sounds contradictory because it is not a heavy duty boot,..it will actually last longer in terms of NORMAL use with steep axle angles. The heavy duty boots will protect it from elements better but will rip apart when overstressed by the axles over and over again in NORMAL use. It's basically a tradeoff....a good boot with clean grease for a long time,...or a tough boot with grease for a shorter length of time.
Good luck