Not sure if I would put Northwestern or Stanford in the Ivy League league ( ), but I would say Northwestern and Stanford are on the same level as each other. Of course it depends on what discipline/major you are talking about, but as far as academic prestige goes, they are similar. Nebraska is a good two or three notches below those schools academically, sorry to say it.
Yeah I didn't mean to imply that they're Ivy League, but they're the closest thing to Ivy League status.
Dont forget about West Virginia and their great academics