Note that the last picture has my neighbors *healthy* plum tree overlapping mine.

I'd repost over at the Fruit & Orchard forum, more experts there and the forum is fairly active.

Just offhand I'd remove some of the pavers surrounding it and examine the soil--it may be too wet--surrounding a tree with concrete is not natural or good for the tree. Another thing to do is examine the trunk and branches very carefully for signs of borers--you will see tiny holes.

But again, probably better help at the Fruit & Orchard forum.

Too much water? Dig down about 4"-6" just before it gets watered and see what you have...dry or wet. Expanding/splitting bark is often too much water. Can't tell from the photo if it is planted at the right depth. The crown of the tree trunk should be slightly higher than the surrounding territory. Agree that it needs more air and that the pavers are too close. It is probably a lost cause, but if you check the roots you will see what is going on. Too wet or too dry. Too wet and they will all be rotten. Too dry and they will be shriveled. Then you can change your schedule accordingly. If it dies, you can plant another one and know better how to care for it in your specific location. We all go thru these (expensive sometimes) learning experiences, but you will be a better gardener for it. Wishing you success.

-Babka