Plumbing problem! Tub drain gurgles when toilet is flushed, and bathroom and kitchen sink drain slowly...?

Okay, so we moved into a home built around 1950, I have no idea what, if any, plumbing has been updated. All pipes under sinks are pvc type. Here is the thing, the tub gurgles when the toilet flushes...happens on and off. The bathroom and kitchen sinks drain slowly. I have cleared the pipes under both sinks, so the problem is past that area. The washing machine drains fine, as does the shower. So, do you think Liquid Fire poured into the slow drains will work or is the problem connected to the gurgles in the tub? I really would love to not have to snake the main line...considering this is my first experience with plumbing. Also, we literally have zilch money to hire a plumber at $75 perhour minimum...would like to take a diy approach is possible...