Could be a lack of rotation, a dropped kick during some phase of the stroke, and/or a lack of shoulder strength. For the shoulders, try Y-T-I exercises with hand weights, light enough (10lb??) for a high rep count. On technique, check out the rotation, front quadrant stroke, and beast of a kick by Katie Ledecky. (You don't need a kick like that, but some kicking helps.)
https://www.youtube.com/watch?v=zEDHGczfDwI

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Strava

My $0.02 is that if you can swim 58 for an IM, then it's time to add other strokes.

A little butterfly should fix that shoulder / lat weakness in a hurry. Maybe a little breaststroke thrown in for good measure 😜

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DFL > DNF > DNS

You need to rotate to correctly engage the lats, rotate from the hips. But it's not just rotating, there is a subtle movement that ensures the lats are engaged, struggling to describe it especially considering I'm on land. Try sitting, put one arm up in the air, then extend it right up towards the ceiling, you'll feel tension in the lats, that's kind of what happens in the water. You should feel the lat take the load, if it doesn't play, around with rotating and really extending your arm out, even if it's exaggerated until you feel it engage.

Look at how level Ledecky is during the drive phase of the stroke. Her torso is perfectly parallel to the lane rope.

My guesses are the 00:58 is wetsuit time and the sore shoulders are from muscling the pull. I get the same when swimming with the TSUN team and I try to keep up, with my lack of kick prowess.

Coach had us do this: 400M : 50 Pull Buoy, 50 Free, 50 Pull Buoy, 50 Free etc.

It was really good because you can contrast both back to back, and see what your kick is doing or isn't
Also do the golf ball in each hand swim to take off the pull and ensure there is no push downforce.

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Thanks for the hints chaps, yes I think you are right, lack of rotation.

I'll try and work on this a bit more, and stop muscling the pull.