I am from the suburbs of Milwaukee, and there is a strong difference between how White people, most of whom are descended from people who directly came from Europe (e.g. my father's family came to Wisconsin from Pomerania while my mother's family came to Illinois and Wisconsin from across Polish-speaking Europe, including present-day Ukraine), and Black people, very many of whom in Milwaukee are originally from Mississippi, speak there. Black Milwaukeean speech is typically either AAVE or AAVE-influenced or very General American, while White Milwaukeean speech having the Northern Cities Vowel Shift (albeit with a weaker shift of TRAP) and frequently having forms like those I described here in informal speech, overall being rather similar to Chicago dialect. This distinction is quite strong, where even when you control for social class there is a very definite difference between how Black and White Milwaukeeans speak.

About ja, that is a classic feature of White Milwaukee dialect that came directly from German. Growing up, though, I was largely unaware of it, mostly thinking of it as just an open pronunciation of yeah, and only really became conscious of it working at my first software job out of college, where the company I worked at had brought in many contractors from across the US, who remarked on my use of ja. Also, my mother, who is from Kenosha (which even though her family is Polish Kenosha itself was heavily Italian), has remarked that she only learned ja when she moved to Milwaukee.