Now it's knocking

If a corner is sitting too low, it sounds like a coil spring has broken. Depending on how it is sitting, a sharp end can puncture the sidewall of that tire. If you can't tell where the spring is sitting, it's probably best if you have it towed, but if you've been driving it like that without the tire blowing out, you might make it another six miles, but I'd go slowly.

If the boxes were no heavier than passengers, there's no reason the suspension shouldn't be able to handle the load. Springs have a coating to prevent rust. Once that coating breaks off, it's only a matter of time before the spring breaks. A heavy load or hitting a big bump won't CAUSE the spring to break if it's in good shape, but it can be the final straw that causes a rusted spring to break.

Replacing the springs is not a real big job. It is basically the same procedure as replacing the struts. There are now also what's called "quick struts" that are new struts with the new springs and upper mounts already installed. You'll still need an alignment after they're installed but you won't need the spring compressor to transfer the spring to the new strut, or in your case, to compress the new spring to install it on the old strut.

New struts, last time I looked, cost somewhere around $40.00 to $60.00 each. New springs should be around $100.00 a pair. A four-wheel alignment typically costs around 50 bucks. The best place to start is by having an inspection done at a tire and alignment shop. That will identify if the spring really is broken.