freeze followed by sudden thaw damage:
boring insect damage:
sun scald damage:

Any of the above conditons could be affecting your tree.
An arborist should be able to determine which is affecting your tree.

A situation, when the tree has broken dormacy in the early spring, of the trunk being frozen after the tree's sap has begun to flow; followed by a sudden thaw, often times is found to produce cracks like you have described has manifested on your tree.

Even though you are only now noticing the crack, it's possible that the trunk could have been damaged since early spring?

Otherwise, if you are certain that the crack only showed up recently, it seems more likely a situation of a boring insect, which could cause such bark damage. You stated that the tree shows signs of being stressed from its exposed location, and that the tree appears to stress the most in summer heat. Most trees that are stressed seem more easily invaded by boring bugs and insect infestations.

If you do not want to hire a tree specialist; the best thing you can do if a boring beetle, insect, or other such bug is what has caused the bark damage, would be to buld up several 4 inch high rings of raised soil around the tree's root-spread area. When finished the area under the tree should look like multiple broad bottomed trenches, each a little further away from the tree's rootball. Those trenches should encircle around the tree's trunk, and spread about two or three feet beyond the tree's dropline.

Once you have created the trenches, apply Bayer's systemic tree and shrub, insect and boring bug prevention product.

This fall is the best time to apply the Bayer product. That way the tree has enough time to take up the right amount of product for bugs that try to attack the tree in the following spring and summer to not find the tree as a hospitable place, or to be a good food source for them.

Once the cause of the trunk damage is resolved, then patiently allow the tree to heal its trunk damage. In future winters and early spring times, protect the tree's trunks from freeze thaw damage; in each fall give the tree the systemic Bayer treatment; then, in each mid-spring you could help the tree further by installing trunk wrap around all the lower unbranched trunks to prevent sun scald.

After all that, if the tree still seems to stress during it's summer heat exposure, you should consider planting an item that is large enough and planted in the right location, to cast afternoon shade on the JM. The best would be a dappled shade which is cast onto the Jm from 2pm to 5pm. If there is no place to plant such a large shrub or shading tree, then at least consider setting up a system of shade cloths which are positioned to cast the needed afternoon shade in the hottest part of the summer.

All that, must seem to be a lot of work, If you do not want to put forth so much effort to help your tree heal, then your other option would be go ahead and treat the tree with the Bayer product described above, but then to wait until just prior to when the JM should break dormancy next February or once Winter's snow melts. If this time usually accurs in your area some time in late February or early March, at that time dig up the tree and move it to a location where it will always, no matter how tall it grows, have the right sun/shade exposure it needs. Then plant a same-sized kind of tree which can handle the sun exposure, which the slope, where you dug up the JM, provides.

Your JM, when it was youngand shorter, might have gotten the kind of shade it needed. More recently, though, it may have out grown the proper shade it needs, which the slope might have provided for the tree, when it was a shorter.