Do you have to exhaust all of your UAW internal remedies & appeals before you can hire an attorney?

You can hire an attorney at anytime

I agree with the first answer, that you may hire an attorney at any time. More specifically, you may be asking 1) whether your union has guidelines for its members about this, or 2) when it would be most beneficial to hire an attorney. Without seeing the bargained agreement for your unit, I cannot say about any guidelines or restrictions. But as to when it is beneficial, an attorney could help in many ways throughout your internal appeals process - at a minimum, one could even just assist you and your union reps identify legal issues and formulate better arguments.
Ultimately, you will only know if you contact an employment and labor attorney to sort through the issues of your situation in detail.

Best wishes,
Andrew