Hal,
I experience what you describe. My speech with family and close friends is much worse than with students, co-workers, strangers. I think I subconsciously make a concerted effort to articulate carefully when speaking with "others", but when with family I just am what I am! My speech difficulties are apparent with both groups, but much more so with family. I am not sure if it is because I am usually tired when with family, or talk more, that it shows so much more with them. I think it is probably about being relaxed and not feeling the need to put up a facade and to think about every word I say so it all just comes out, garbled or not.
While I have discussed my speech issues with my neuros (a year ago) they did not "hear" it, but at least acknowledged it, suggesting it may have come from a stroke. Since it is not improved, but only worsened, I think I have reached the point where the impaired speech, even when careful attention is paid to articulating, is becoming obvious to any listener. I assume it will provide another clue to the as-yet-unnamed mystery disorder that I have the pleasure of living with.
Lydia