Load Balancing multiple IPs with the same gateway.

It may sound strange, but we've got 5 addresses on one provider,
with a maximum bandwidth cap on each, so we'd like to load balance
them.

I've looked at the advanced routing howto, section 4.2.1, but
that technique presupposes that each gateway will be different, or
each interface.

In my case, the interface and gateway are both the same; the only
change is the source ip address of the ethernet card.

Just messing around on my own I haven't gotten shared routing working.

Right now I'm doing a hack of just changing the address ever 15 minutes.

Anybody know if this is even possible? And if so, how?

Thanks for your help.