Leslie, you are an angel for taking them on.
Here are some MI GSD rescues:

Kari Selinger
Marshall, MI 490688
[email protected]
616-781-9190

Andrea Kyriacon
Detroit, MI 48239
313-277-0908
[email protected]

Gail Gray
Bay City, MI 48706-9316
313-475-3570

I don't know how up to date the rest of the list is, but karipets is active, as far as I know. Maybe she could link you up to an all breed rescue group.
You might be surprised at how quickly they may get adopted. My first foster, Gaylord (may he rest in peace), was 7 yrs. old and limping badly and was still adopted in a few short weeks. Bandit is 5 yrs. old and adopted in a flash, too.

I know what you mean by four big dogs in the house!
That is rough. Let us know how it goes, please and good luck!