Hey,
I have 1981 GMC Rally STX, in good condition, but all 4 speakers are no good. I bought a new set of 6x9's for the back, figuring I'd tackle those ones first, but I'm having some issues.. I think I have to take the whole inside panel off to get at the speakers, and I'm a litle timid about breaking something.. Anyone have any tips, or should I just take it in and get them installed professionally?
Thanks.

I have a almost-identical van. The inside panels unscrew quite easily. Just take your time, start with removing the visible screws and remove the panels. Sometimes some screws may be hidden.
It's an easy job. Just keep track of how the panels came apart so you can put it back together again.
But do not be surprised if your original speakers are not 6x9's. My rear speakers are not that size.
Depending on the interior layout, the factory speaker cavities may be a different size. If yes, the 6x9's can be made to fit, but a pro should do that. Or new original-size speakers are likely available.

Well you could do what the person who owned my van before me did, just buy two home stero speakers and screw them to the panel lol, but seriously i wouldnt do that, MagivRat is right, those panels come off quite easily, its just a big puzzle, that if you take you time will be easy.