Re: She consistently juts her lower jaw forward giving her an underbite just for these sounds. Her tongue is visible between the upper and lower jaw. We have worked on jaw stability and tongue placement but this posture continues to occur during drill work and conversational speech.
I am not sure what jaw stability has to do with what the child is doing with her jaw. She does the posture with sh, ch, j, s and z. Does she have a correct position for all other phonemes?
I would start by having her understand jaw movement by feeling it. Ask her to place a finger on her chin when her teeth are in a normal closed bite position. Then demonstrate jaw protrusion. She can place a finger on your chin as you do the demo. Ask her if she can do the same with her jaw. When you feel she understands what she is doing, explain that when we say sounds our jaws don’t move forward (as she is prone to do). Then do some phoneme practice (she should say the phonemes with her finger on her chin):
Examples:
/t/—–/s/
/k/—–/s/
/d/—-”sh”
/n/—-”ch”
etc
She will need your feedback as she practices. Once she can produce the target phoneme without the forward jaw movement, work on it in isolation for consistency then CV, VC.
Using the targeted sounds in conversation should not be expected until she is able to repeat sentences with correct production of those sounds. Take you time—-a month away is realistic.