There are several questions on this site that deal with the correct pronunciation of "ll" and "y" but despite sounding like a duplicate, this question addresses a slightly different issue.
I have read almost universally that "ll" is pronounced as the "y" of "yellow" in Mexico. However, I also read someone saying that at least in Guadalajara they use a mix of two pronunciations (the "y" of "yellow" as well as the "j" of "jello"). He goes on to say that words like ella and amarillo are pronounced with the "yellow" sound whereas the place name Saltillo is pronounced with the "jello" sound.
Yet another resource gives the "jello" sound as the prevalent pronunciation in DF. Now it clearly seems that both the sounds are used in varying degrees in different regions of Mexico and certain words are pronounced with one sound while others go with the other sound. I really want to know and understand how the Mexicans decide which sound to go with for a given word. Are there unstated phonetic rules that the locals (Mexicans) follow?