Modifying a copyrighted tag line

What is the name of your state (only U.S. law)? Florida

Is it legal to take a copyrighted tag line, modify it, and use it for personal gain or profit. What would I need to do to make it legal? For example, if a company had a copyright on a T-shirt that said "Got Milk?" and I wanted to sell T-shirts that said "Got Chocolate Milk?" would that be legal and what, if anything would I need to do to make it legal?

What you would need to do is either get permission to use your modified version of the tag line from the rights-holder, or make sure your modification of the tag line is so clearly a parody (and a good parody) that a court would see your use as a fair use. The court's opinion would come, by the way, when the rights-holder discovers your use and sues you for infringement or disparagement.

All of law is fact-based and, while I can give you case examples showing you what a specific court has ruled a fair use (ex: Barbie-doll-based noncommercial art was a fair use) and what a specific court has ruled not a fair use (ex: a Cocaine tee-shirt that mimicked the Coca-Cola trademark was not a fair use), this will not really tell you if YOUR planned use of a particular tag line will be seen that way by a court. What the cases can do is detail for you what factors a court will look at when judging fair use.

What you really need to do, if you intend to market products using rights-protected material (and commercial uses, as a note, are not often considered a fair use), is to get permission from the rights-holder or review your specific plans with a legal professional in your area.

As a note: Holders of famous trademarks (think ones like the NFL, Disney, Budweiser) rarely find uses of their marks funny (even the parodies of their marks), and they are not shy about threatening to sue or suing.