Yeah I think the tricky part is the output to the rumble motors is only like 2v peak I think. I'm having a tough time finding anything that will go from 2v to 12-24v. I did find a small step-up transformer that will take the 2v and output a steady 5v but that's probably not enough...
What you want is to use the low power coming from the controller to switch on higher power from a different power supply.
You don't want to step it up using only the power coming out of the controller.
Looking at the solid state relays, it does look like most of them require at least 3v to trigger.
Google "using NPN transistor as relay" and read up.
Also look up "protection diode"
That is what you want to do.
It sounds complicated, but all we're talking about here is running the lead from the xbox controller through a diode (which doesn't let anything move backwards through it) to one leg on a transistor.
Then you'll have a bigger power supply running to the motor with the power lead cut and routed through the other two legs on the transistor.
It's that simple. The hardest part will be finding the correct values that you need. If the transistor gets too hot, you may have to add a heat sink.
Are you sure the motors are only getting 2v? That sounds low to me, but I haven't tested them.
EDIT: I tell you what... I have these parts laying around. Let me know how much voltage each vibration motor lead is putting out (IIRC there is a weak and strong motor) so I can do some testing.
If it works I'll mail you an already built circuit.