http://www.geocities.com/endre_rl/isuzufaq.htm#faq5
If it is not that, it could be something with the powersteering pump. There is a switch in the pump, that when it gets dirty is stuck open. I think the switch is suppose to turn on when you have the steering wheel cranked in one direction. With the switch on it raises the idel speed a bit (but up to 2000 seems a bit much).
Also, I know the 98-99 has a cable run accelerator (00-on have drive by wire). On the 93-97 it has the same thing, but over time there is a rubber dust cover boot on it there breaks down and can get caught causing a high idle. I'm not sure if the 98-99 has the same set up?
Check it out.
-Ryan