I just launched a new site that deals with all and any problems with the 4wd system on Yamahas. I have how to videos for testing the servo, disassembling the servo, replacing and purchasing the servo (motor only), testing relays, and testing the push button switch.
I also have a link to download the 4wd testing portion of the service manual.
Check it out at 501parts.com - Yahama Servo Motors
Let me know if I can help.
Heres a few of the videos and pics
http://www.501parts.com/files/stacks_image_252.png
http://www.501parts.com/files/stacks_image_250.png