I'm not a professional mechanic, but this is how I modified our 1999 Suzuki LT300F King Quad so that "Diff Lock" is available in all three ranges, SuperLow, Low, and High.
** I advise AGAINST using diff lock in High range, as the steering would be too tough and dangerous. **
However, we use our King Quad in Low range to plow snow and having the front axle locked should help a lot.
Total time to do this job was about 50 minutes, and that includes videotaping the whole process. I know this modification will work on other model years, but no promises...

If this mod works for you, or if you'd like to see more of our King Quad and the various things we've done to it, PLEASE COMMENT! Thanks!

I've installed custom plow lights, "start in any gear" mod, home made winchpowered plow lifting mechanism, auxiliary LED taillights, etc...
Start in any gear mod: http://www.quadcrazy.com/atvforum/suz...

No guarantees if that will work for you. I know that wire colors rarely match throughout model years. Snip at your own risk!