Unfortunately galinas it sounds "mostly" dead. Your scratch test shows that you might see some succors come in around the base of your tree. The only issue with those is they sometimes don't give you the fruit that you want.
I would suggest watching the tree this year, go ahead and do your drainage ditch just to make sure. You might get lucky and see some late season growth pop up. If nothing happens then you will want to remove the dead tree. It can promote some big issues with new trees, like pests that can severely damage your new tree. Now that you know what sort of drainage issue youre having for a new tree prepare your drainage first. If you can scout out a new spot to see if the drainage works better there, if not just do a drainage ditch for it. Hope that helps!! Good Luck!

Unfortunately, my yard is tiny - and I have big ambitions) What I am planning to do, if I have to plant a new tree, I will bring a couple yards of topsoil and will make an artificial hill - I already made one for my new apricot tree and it went through this harsh winter alive. Originally our lot was very sloppy, so construction workers added about 2-3 feet of complete junk - gravel mixed with clay, huge rocks, bricks and so on... And then covered it with 2 inches of topsoil. So when you dig a hole in it and fill it with a nice soil, you getting pretty much a teacup, which will retain the water, because the junk soil around the hole has almost no drainage, despite it is 50% gravel. What I don't know though, is how the roots of the plum tree are located. I guess feeding roots should be in the top layer, because it is very easy to damage a small root when you cultivate around the tree. Those roots should be OK on the hill. But not sure how anchor roots will do - do they go deep or wide?

This post was edited by galinas on Tue, May 6, 14 at 16:22

Update to my post. The tree is really dead. I shook the trunk today and was surprised that i see the roots moving in the ground a lot... Believe or not, but I took 7-8 ' high tree out of the ground without any tools - just pulled(. The root ball was not bigger then 1.5' in diameter, just two roots where 2.5'. All the roots a brown inside.

I killed my weeping cherry (flowering one) a few years ago by planting it on the wet spot where my yard dips. It grew well for the first 3 years and the next spring, it never woke up.

Learn if afterward that among fruit trees, cherry (even flowering one) really does not like wet feet. It's a $55 lesson for me. No tree goes into that spot again.