Unfortunately I am going to interject a little reality here. Everything mentioned will work, sorta. A resistor will limit current, and that will cause the voltage to drop but it would have to be a huge, really, a big one, to handle the current for servo drives. A proper one would probably cost more than a new power supply.
Voltage regulators: Linear, (linear technology) does make "linear" voltage regulators, like the 7805 (a positive 5 volt regulator).
I think He is confusing the term linear (as in the technology of linear regulators) with the company Linear Technology, oof- confusing.
Ok now to the point. There is really not a good way to do what you would like to do. The suggestion of unwinding some of the secondary is in my opinion the best advice. This is what I would do. It will not change the current at all, that is based on the wire gauge and not the length. Unfortunately this is not a really easy thing to do.
A resistor just isn't an option (sorry Chaoticone
Linear regulators just are not good with high current things like servo drives. It could be done but it would cost much more than the correct power supply.
So I guess the bottom line is put that supply up on ebay and go get yourself the proper supply. I would look for a multi output switching supply that does what you want. Double the max current that you think you will need and you chould be good to go. Power supplys are not the place to cheap out for servo drives.
Chad