* Informal "yeah" and "ya" are often pronounced the same: In casual American English, both "yeah" and "ya" are frequently spoken with the same pronunciation, sounding like a shortened "yuh".
* Formal "yeah" and "ya" are different: In more formal settings, "yeah" is pronounced with a more distinct "eh" sound at the end, while "ya" (as a shortened "you") is rarely used.
* "Ya" has broader uses: "Ya" can also be used as a short form of "you" in phrases like "What's up ya?"
In summary: While the pronunciation of "yeah" and "ya" can overlap in casual speech, they have distinct origins and are used in different contexts.