Rear brakes have an adjuster that you can turn to dial in your brakes. They could be turned in too much. I would pull your drums off, which you may need to find a bolt that threads into the two hole near the studs to back the drum off the brakes.
Once you've taken it off, visually look at each part to make sure you don't have leaks and that springs haven't fallen off. Google 3rd gen 4runner drum photo or YouTube it and you can verify you have everything there. There's actually a video on YouTube showing how to replace the brake shoes. Depending how worn they are you may want to replace them.
The adjuster is towards the top which you can turn so it collapses the brake shoes. Adjust it so you can put the drum back over the brake shoes. On the back side of the drum, facing towards the center of the vehicle, there is a hole where you can adjust the brakes. You may need to remove the rubber grommet to access it. With a flat head a screw driver turn the adjuster until you feel some resistance on the drums from the brake shoes, but don't adjust it too much where you can't pull the drums off.
Again look it up on YouTube which will go more in depth. Also do the tundra upgrade, its well worth it.
Edit: Oh and anytime you work on bakes, make sure you start on one side, complete it and verify you put it back together right before moving to the other side.
Its easy to forget how things go back so jumping to the other side to look is a life saver
