I couldn't find any reference to a PB&J milkshake on the Moms Panel site, but I did find several references to this on other Disney-related websites. The PB&J milkshake is a specialty of the 50s Prime Time Cafe at Disney's Hollywood Studios. On the website allearsnet.com I even found a recipe for it:
1/4 cup creamy-style peanut butter
3 tablespoons grape jelly
1/4 cup milk
3 cups softened vanilla ice cream
First mix the peanut butter and the jelly together in a small bowl, then mix all the ingredients together in an electric blender.
I have never tried this particular Disney treat, but it has definitely risen to the top of my "must do" list for my next trip to WDW.
I found many other fun beverages at WDW when researching the question
HERE.
Happy tasting!