I found this today for the foreman 500. I don't know if it would work for the Pioneer 500....
"Hello there gentlemen,after countless hours of trial and error I have finally figured out how to lock the front diff. without effecting the steering.There is no welding involved.If any of guys have ever been inside the diff. this will be easy for you.Once you take diff. apart take out your ring gear assembly.Take out all the bolts on the ring gear.Before you take it apart hold it together then flip it over.This will keep all the parts from falling out.Take the ring gear off and you should see the gear on top and two washers.If not, then it is stuck inside the ring gear because of the oil.It should pop out easy th
ne washer should be flat and the other is coned shaped.The reason it is coned shaped is because it is a spring.When the front diff. is under stain this washer flattens out and allows the gear to slip.What you do is go to your honda shop or go to
CheapCycleParts.com - Cheap Cycle Parts and order another coned washer.The washer is called a side coned spring, part number 42461-HN5-671 for 10 bucks.Put the two coned washers together in the same direction not opposite of each other.Then just put the two coned washers and flat washer back in and then put your ring gear back on.Flip it over then put all your bolts back in.This added washer keeps the washers from flattening out and keeps both wheels spinning.You have not changed the function of the diff. so therefore it will continue to work as it should.Mine turns just like it should and pulls all 4 wheels when in 4wheel drive.I dont know if honda makes a locking diff. but I have not found one yet.Besides that im sure it cost a pretty penny.If you have any questions about this just reply and I will help you anyway I can Thanks......"