From what I've heard, the plastic (orange) impellers were used up till early '97. But there's no way to tell for sure without removing the pump. If you're not sure, don't play Russian Roulette with it, just change it out for peace of mind.
Mine was already changed out and has about 30k on a metal impeller, but I figure I should replace it again while I have everything apart . . . I'll be flushing the coolant, doing the fan delete mod, switching to an aluminum thermostat housing, and replacing the upper & lower radiator hoses. After this, I should be good to go for a while.
[Edit]: Ahhh, found one on eBay for $41