Its most likely the relay. You can buy a new relay box for about $200 from isuzu or if you can solder or arent affraid to learn you can buy some relays from Mouser for about $5. They dont carry the oem fujitsu relays anymore and last I checked they were out of the NEC 31000s relays but others have used NEC 22111s relays with no problem.

Here is a write up I did on replacing mine..
http://forum.planetisuzoo.com/viewtopic.php?t=5608

--Dave

1999 Rodeo LS 3.2 4x4 5 Speed
32x11.50-15 BFG T/A KO on US Wheel 94 series
3" Lift - Rancho 9000x
Rear No-Slip Locker
Flowmaster 50 series SUV 2.25"

There is a very fine line between "hobby" and "mental illness." - Dave Barry