SQL Server 2016

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declare @t table(id int identity, col1 varchar(10), col2 int, col3 bit, col4 int);

insert into @t(col1,col2, col3, col4)
values
('ABC', 123, 321, 111),
('ABC', 333, 321, 123);

SELECT
  aId = a.id,
  bId = b.id,
  v.columnName,
  v.value1,
  v.value2
FROM @t a
JOIN @t b
    ON a.id < b.id
 -- alternatively
 -- ON a.id = 1 AND b.id = 2
CROSS APPLY (VALUES
   ('col1', CAST(a.col1 AS nvarchar(100)), CAST(b.col1 AS nvarchar(100))),
   ('col2', CAST(a.col2 AS nvarchar(100)), CAST(b.col2 AS nvarchar(100))),
   ('col3', CAST(a.col3 AS nvarchar(100)), CAST(b.col3 AS nvarchar(100))),
   ('col4', CAST(a.col4 AS nvarchar(100)), CAST(b.col4 AS nvarchar(100)))
) v (ColumnName, Value1, Value2)
WHERE EXISTS (SELECT v.Value1 INTERSECT SELECT v.Value2)
FOR JSON PATH;

WITH allValues AS (
    SELECT
      t.id,
      j2.[key],
      j2.value,
      j2.type
    FROM @t t
    CROSS APPLY (
        SELECT t.*
        FOR JSON PATH, INCLUDE_NULL_VALUES, WITHOUT_ARRAY_WRAPPER
    ) j1(json)
    CROSS APPLY OPENJSON(j1.json) j2
    WHERE j2.[key] <> 'id'
)
SELECT
  aId = a.id,
  bId = b.id,
  columnName = a.[key],
  value1 = a.value,
  value2 = b.value
FROM allValues a
JOIN allValues b ON a.[key] = b.[key]
    AND a.id < b.id
 -- alternatively
 -- AND a.id = 1 AND b.id = 2
WHERE a.type <> b.type
   OR a.value <> b.value
FOR JSON PATH;

JSON_F52E2B61-18A1-11d1-B105-00805F49916B
[{"aId":1,"bId":2,"columnName":"col1","value1":"ABC","value2":"ABC"},{"aId":1,"bId":2,"columnName":"col3","value1":"1","value2":"1"}]

JSON_F52E2B61-18A1-11d1-B105-00805F49916B
[{"aId":1,"bId":2,"columnName":"col2","value1":"123","value2":"333"},{"aId":1,"bId":2,"columnName":"col4","value1":"111","value2":"123"}]