How is your overheating issue?
Take a look over at Townace answers. Its a good place to start with these things, although you have to be ready to dig a bit.
http://www.townace.plus.com/aanswers.htm
Does the van have the original viscous fan? Could be the coupling/clutch is worn out and not operating properly. Also may be worth re-coring the radiator(s). (Note: not sure what year toyota introduced this, but my van is a 1990 and actually has 2 radiators. One vertical just in front of the engine and another, horizontal, a touch in front of the vertical one. This horizontal radiator has 2 (if I recall) electric powered fans. May be worth ensuring these are also functioning.
Good luck.
Matt