These amphibious 6x6 machines come with significant compromises.

• Looking at the specs, the only suspension is in the tires. Not surprising considering that you'd want to make as few holes as possible in the lower body shell.
• The steering is done by differential braking or even reversing. That means it turns on a dime. But turning will tear up the soil.

The argo 6x6 750 HDi has 9.5 inches of ground clearance. That sounds like a lot. But how deep are the ruts in your driveway?

And even with one of these, you can still get stuck.

The main problem with your soil is that it's really soggy. So every time that you use an ATV or 6x6, you'll churn up the mud and the ruts will get deeper. In the end the mud will probably win, unless you keep dumping truckloads of gravel in your swamp.

Getting a 6x6 with the optional tracks might help. Those would help spread the load over more area.

A hovercraft doesn't tear up the soil and can traverse almost any terrain, but they're not all that great for hills. And they are notorious gas-guzzlers, and noisy.

Winch and a hovercraft. www.hovercraft.com has a variety of sizes, just delete the propulsion. Or rather than a winch, a rope tow, with a return pulley at the other side of the creek.

Or build a "barge" with large diameter inflatable rollers, and drag it across with a winch.

Or a sled?

What about a heavy duty pulley system? Something similar to what people use for clotheslines, but able to hold more weight, then you could hang a platform from it to put the straw bales, etc on it.

My mother has a pulley system for her bird feeders that probably holds a few hundred pounds when the feeder are all full (she's pretty serious about her birds); I'm not sure what your heaviest loads would be but at least a couple hundred pounds should be do-able.