I took a look at the fbx………..seemed complex to follow as I saw there were unnecessary bones unless you wanted to make an animation of the digger?
one thing I would already suggest is to follow havocs tutorial on his naming conventions it's much easier to understand which name relates to what it's related to..... but whatever helps you....
another is I would suggest you bind the bones in wireframe view it looks like the binds are very scattered and might be a reason it cant import... but when you bind make sure you are clicking on the specific shape and not a section of the entire mesh..... in case that's what you did otherwise skip this...
what I did was make a root bone which represents the object and all it's parts and now this bone I bounded it to the entire object. Afterwards I decided to make a driver bone needed to spawn an animation and thus a model and this bone I did not bind anything to it.
the yaw and pitch bones are related to the parts of the object I wanted to have turn (turn left and turn right - Yaw) and then what parts I wanted to look up and look down - pitch. the locations of these 2 bones is specific too because any part of the object binded to these bones will turn in the bones axis. So if I bind the wheels to the pitch the wheels will then rotate in the pitch bones axis …..
the wheels then are in the respective places and named accordingly and every name has b in it meaning bone. it is easier to know which bones correlates to what area of the object so when you get into coding and the sdk part everything is right there in front of you easier to know which is witch. and the objects pivot point is at 0,0,0 so that when you spawn said vehicle the spawn is at that 0,0,0 mark and not half in the map or something else
done in 3ds max... sorry not blender...