I've seen the used 98' heads go a lot cheaper, than the 99' & up. Are there any differences? If so, then what are they?

Thanks

 

One difference is the way the valve covers bolt to the head and the way the coil packs are attached to the valve cover. The coil pack on 98's bolt directly to the valve cover as where the 99 and up bolt the coil to a coil pack rail and it bolts to the valve cover.
Tom
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Also the '98 valve cover bolt onto the heads with bolts around the perimeter and the '99 and newer are center bolt.