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What's the difference?

Is tying forcing consumers to buy something and bundling an attempt to monopolize under S. 2 of the Sherman Act?

tying = actual blocking of the monopoly product, unless you buy the commodity product
bundling = incentive to buy the monopoly product, if you buy the commodity product

bundling is like tying junior. In both cases, the idea is the customer is somewhat or actually forced to buy the commodity product, because they want/need the monopoly product.

Exactly. It's basically optional (bundling) v. mandatory (tying). In a bundle it is still possible for a consumer to buy the products separately if they don't value the bundle enough. By contrast, the whole point of tying is to eliminate that option: a consumer can't buy the monopoly product without also buying the competitive product.