Could you explain what you mean by in between her jawbones? And about how big is the lump, hard or soft, tender to the touch? I think you need to give many more details. We still may not get a consensus, though.
If you are scratching under the chin...half way between her mouth and neck...it is in the center between her lower jaw bones...and is a bit smaller than a dime. It moves around kinda like a fatty cyst or marble...it is not soft and squishy...but not hard as a rock either. It doesnt seem to bother her. I dont know if it is a lymph node or just a little csyt...she has had it for a while and she just turned 1.
I can't feel anything like that on Luca who has an easy chin/neck to palpate. It could be many things but a salivary gland cyst seems possible. The fact that it moves and is not rock hard sounds good to me. At any rate, I think you should have your vet check it sooner rather than later. Some conditions become more difficult to treat if left untreated.
It is really hard to see that on the doodles with all the fur and the sutle differences in the thickness of the wiskers. I probably wouldn't have realized that is what it is but since Jazzy the lab x has it and it is very obvious on her I knew that is what Neely had too.