Found a crack on top of plastic reserve coolant tank.

[Mrkentee]

Probaby. I just wanted to say, since you'll be replacing it. Dorman makes an aftermarket one that's not as expensive. From what I heard, I wouldn't recommend it.
On mine, I got prices as high as $585 from Rush Truck Center in Denver, since I was up there. (Wasn't leaking. Threads on cap were screwed up) I called one of my dealers back home in New England and got a much better price. Long story short, the parts guy at KW in Kansas City, Mo was able to sell it to me for $314, with no sales tax.
So my point is, the prices on these things are all over the place. Shop around.