I would probably repot ASAP if it was in ANY BRAND of store bought bagged soil. If so It's probably got severely compromised roots!

If it was in properly made 5-1-1 or Gritty Mix, I would pop it out of the pot and check the roots for compaction, foul odor, mushy brown roots.

Even in Gritty Mix for 3 years, one for 4 years, my JMs show of slight loss of vigor, but no leaf drop or dieback I will do a full repot/root prune in the spring.

I will take it out of the pot this weekend to check on the roots. Would it be risky to plant it in the ground or should I wait until Spring?

Now is a good time to plant. Planting in the Fall allows the tree to acclimate to it's new environment when the temperatures turn cooler.

I concur, if you wish to plant it in the ground, now is a good time. You don't need to root prune (unless there are dead and rotting roots) if you are going to plant it out.

If you wish to keep it containerized you should check it out now and repot in early spring, before it leafs out.