Been there, done that, 4 wheeling and with a tractor with a 5' brushhog on the back. First thing to do is to get that box blade off. That is a pretty heavy weight at a moment arm pushing down on the back of the tractor. Since the rear wheels are down in the ruts, the tires are going to have a hard time lifting that weight as well as the rear of the tractor to crawl their way back on top. Instead, they just dig into the face at the end of the rut and make a trench. Take it off and manhandle it off to the side and pick it up later when things dry out.
Next, as suggested, use the bucket curl hydraulics to push you back while backing slowly. You do NOT want to spin the wheels, you want the rear wheels to crawl back up on top.
Here are a few questions for you:
How far behind the tactor is the nearest solid object such as a tree?
Do the rear wheels on your Kubota have holes through the rims big enough to pass a chain or strap through(I couldn't tell from the pics)?
An old 4X4 trick is to use the tires themselvs as a winch but you need a low range(which the tractor has) and a locking differential(which your tractor may have) or use two lines, one to each wheel. You pass a tow strap cable or chain through the wheel from side to side(see attached drawing) and fasten it to itself in the middle of the tire tread. To this connect a longer strap/cable/chain and run the opposite end of this back to a solid object(take out all the slack in the line). The large truck you mentioned setting on solid ground might work if there are no trees close by. Failing that you could dig in an anchor to attach to. I have seen railroad ties, spare tires and even large boat anchors buried in the ground to use as an anchor point.
At the slowest wheelspeed your tractor is capable of, start to back up. The tow line will be drawn down into the mud around the bottom of the tire untill tension forces the tire to start to roll back up the line. Here is where you need that locking differential or a second tow line rigged to the other wheel. As the one wheel takes a strain on the tow line, without a locking diff, the other wheel which has less traction will just spin. If you have independent brakes, you can also apply the brakes to the side that starts to spin which will send torque to the wheel that is pulling on the line.
The main drawback of course is that the solid object needs to be directly behind the wheel as any angle will cause the line to slip off the inside or outside of the wheel. A short strap or section of chain around the tire with the long line connected to this works the best. That way if(when) it slips off to the side, as the short attach piece emerges at the front of the wheel, you can easilly disconnect everything and re-attach again at the rear of the wheel. The times I have used this trick, I have actually relied on the line comming off either the inside or the outside and I wind up moveing a few feet at a time and re-rig. This is much easier to untangle when you are finally out.
Using this and my loader, I got my 3000# tractor out last fall after I drove into a swampy spot while brushhogging. The rear wheels had to climb up about 14-16 inches to get back on solid ground and they just couldnt do it. I put a small chain through the wheel and hooked a large chain back to a tree, engaged the difflock and creeper gears and the rear end crawled right back up the chain onto solid ground. Of course I had to get the 5' hog out of the way first(took me longer to wrestle the mower out of the way than it did to get unstuck). I was very glad I had a 30" fork bucket on at the time. With the forks straight down, I was pushing the bucket frame and loader arms into the muck before I was hitting anything solid enough to push against.
Good Luck.
