There is simply no reason for those muscles to be tensed, because they do not assist you at all in singing notes.
The question is why are they tensed? Knowing that they are not used in singing should allow you to relax your throat, especially the swallowing muscles. They actually work to run food down the exophagus, and you sing from your trachea.
Can you isolate the reason why you are tensing these muscles? Eliminating the urge to tense them is a start. If you have a lot of general tension, you may need to do some relaxation techniques, as in yoga or meditation, to learn to relax in general before you sing. Then maintain a relaxed state when you sing.
It's important for you to learn to relax your throat.
It's also important for you to use proper support to take the load off your throat and transfer the work to your diaphragm. Your throat will never be able to sustain high notes if you put the load on your throat and neck. Your constrictor muscles are for swallowing. You are not drinking or eating anything when you sing, so tensing them has no purpose and is completely unrelated to singing. You are introducing that tension and you must be the one to stop doing so. It's not a part of your vocalizing.
You are mistaking the mechanics of singing.
It's as if you are driving a car and putting your foot on the brakes as you apply the other foot to the accelerator to get onto the freeway. What do you do in that case? You stop putting on the brakes. I can't tell you why you are tensing muscles that I am asking you to relax. You will have to do a little tension-checking and learn to control your own body. It will be very beneficial to your singing.
I'm not trying to be mean or cynical with you. I'm being very straightforward. Feel free to put a video up so we can try to help you sort this out. We're on your side, here. It's just difficult to detect WHY you're doing what you are doing.
Don't give up. Let's just get this figured out, so you can proceed with your vocal progress.
All the best!
Thanks for the update. I'm glad you were able to get it sorted out and you told us what the solution turned out to be. Sometimes you just have to bounce problems off of someone else to get all of the issues on the table so you can get to the solution.
Good job at eliminating that tension. Others can learn from this. Tension does not belong in the throat or neck. It can be helpful in holding back the diaphragm.
All the Best!