The lowest rate will likely be through refinancing your mortgage to include the amount of your loan — even thought the interest is technically not tax=-deductible on this portion of the loan. The uncle could do the same.
Or, plan B, contact either Credible.com or SoFI.com to see whether you can refi through them. I agree that rates are insanely high if you stick with Navient.
And, though you didn’t ask, maybe it’s the burden of these loans that is adding to your daughter’s depression. Time to get her some help, because this is really the biggest problem you face. Maybe it requires meds, or maybe counseling. But her entire life is in front of her. And if you can deal with the debt burden, which must be weighing on her conscience, maybe it is the first step to a more positive attitude. (Yes, I’m way out of my lane — but I see so many grads burdened with debt and it seems to put a real cloud over their future) so you’re not alone.)