If you are losing a gallon per day of coolant you should be able to see a major leak somewhere. Have you let the engine run and gone underneath with a flashlight?

If you plan on replacing the radiator, don't waste your time with a used junkyard one.
Also plan on replacing all of your coolant and heater hoses, and thermostat. Another thing to check is your "hose of death".
Do a search on this website as well as the one in the link below:

http://www.toyotavanpeople.com/forum...adiator#p56468

what's the radiator-like thing forward of the radiator itself?
That should be your a/c condensor.