As a previous commenter alluded too you need a proper backpack or at least a properly configured backpack. GoRuck is the defacto on this but any bag made of serious nylon with olympic plates and internal PALS MOLLE can work as you can just strap the weight close to your back.

The weight needs to be high up on your back and as close to your body as possible (there is some debate on this but in general that is what most people believe).

Also I don't have a citing on this but I'm fairly certain most people have back problems not from carrying heavy weight on their back but rather sitting down too much and picking up the weight incorrectly.

Rucking is more than dropping a kettlebell in a JanSport -- you will want a rucksack that is designed to properly support a balanced load on your back.

I recently picked up a GoRuck GR1 [0] -- it's an amazing bag for everyday use. They also sell weighted ruck plates that fit the rucks and keep the load close to your body.

[0] http://www.goruck.com/gr1-explained