The last two days whenever I've been taking a shower, midway through the toilet will start making a gurgling noise. I've heard this could be due to two things, a blocked vent or a blocked sewer line below where the toilet and shower lines connect. Both times I had the toilet seat down so I couldn't see if air looked like it was being sucked down or bubbling up, but afterwards the water level in the toilet was lower than usual. Water continued to drain normally from the shower while the toilet was bubbling, and the shower and sink drains remain dry when I flush the toilet.

Toilet still flushes like a champ (drains immediately), and I know for a fact nothing's gone down there except waste and TP. Nothing's gone down the sink or shower drain except soap and possibly hair. I'm on a septic system, and the tank has been emptied relatively recently. No trees near the pipes or drainage field.

What should I do/do I need to call a plumber? If there's a partial blockage, is there any chance it will resolve itself on its own?

There's a restriction in the drainage piping that needs to be mechanically cleaned(no chemicals). It won't resolve itself.

If you run water into any of your fixtures long enough, your sewer pipe will eventually fill up, all the fixtures will be affected, and nothing will work properly.

clog in the drain..
find an available cleanout and snake the pipe.
if you want, you can pull the toilet and snake thru there.