Sorry to hear your test drive didn't work
If the motor and central diff is turning, you should be able to see which of the drive axles is turning, as that is where all the motor power is being used - that should point you towards the wheel which isn't working properly.
Re diff fluid, you'll need to get that from an RC car/buggy store. I found some FastTrax 500K weight fluid on ebay recently but as I don't have a working chassis yet I didn't buy it.
You can also get spare "Hobao Hyper" diff kits of eBay, which would allow you to see how all the parts go together without performing surgery
on your existing ones. You will need to get all the gears and shafts out and cleaned up if you want to change the diff oil in them.
Here's an online run through for building a diff:
http://www.twf8.ws/new/tech/car/hobao/h ... step1.html