"O.K., I have a tight jaw. What can I do about it?"
Some years ago, when I was singing in Germany, I went to Armand MacLean-Lanier in Frankfurt for a lesson. Armand was a great character and a fine teacher. He saw that my jaws were extremely tight and gave me an exercise which I still use with my students.
Those of you who deal with the same issue may find it helpful. The exercise is a string of syllables on one pitch. It is done on a comfortable middle register note. and can be continued up and done the scale ad lib.
The syllables are "Geh-gi-geh-gi-kich" (ɡɛ-ɡi-ɡɛ-ɡi-kiç), where the ch is the German "ch" from "ich". Do it with about a thumbs-width of space between the back teeth, and without moving the jaws. Keep a steady flow of air, and good focus (clarity) in the tone. You can also do it with a hand on the jaw to aid the release.
I will never forget seeing a Bulgarian friend of mine (a great mezzo soprano) start her morning warm-up with her hand on her jaw, and a series of slow moving scales.
You can also do this exercise on three note scale patters (12321). Try it, and let me know how it works for you.