Okay, the first pic is a 98 for sure, the hood graphics as well as the tunnel support it. That tunnel on that machine doesnt have the support brace or the wider, beefier runningboards with the squarish edges. I believe in 99 that they went with the supports and the wider beefed up runningboards because they had issues with guys like Blair Morgan completely demolishing them while running snowcross.
That second picture has to be a 99, look at the tunnel support as well as the wider boards with the squared off edges. The 98's tunnel looks much more like the production ZR tunnel.