Not sure

[tmontague]

Visually inspect the rear suspension and axle when trying to make the parts move to re create the squeak. It should have read "visually inspect the suspension to try and see what is causing the squeak"

If it is not happening through the suspension travel than it is possible that it isn't the actual suspension but something else. It's tough to say without actually seeing the car. Make sure the shocks arn't leaking as well.

I'm not too familiar w/ Gabriel parts but at 50k miles they may be nearing their lifespan.

I cannot remember the load capacity of the Yaris but a single 300lbs person in the rear should be fine unless you hit a large drop. It shouldn't bottom out on more minor bumps. If you do hit the bumpstops once in a while it is not a huge issue. I don't know how often you have people that heavy in the back seat, but if you do it often than I guess I can see your desire for stiffer springs.