If the diagonals are not equal, then either of two things is wonky.
1. The side walls are not plumb presently, and when you pull the top plate inwards they will either be more out of plumb or more in.
2. The ends of the front, unsheathed wall are not of equal height. If they were built on the floor with equal length studs, that's not likely. If the studs were nailed in place then more likely.... ?
Check both side walls right at the front corner for plumb and see what they read. This ain't rocket surgery, so you can make it better without too much trouble. :smile3:
If you don't have a come along, drive a substantial stake into the ground about 8 ft away and brace a 2 x 4 against the wall at the top to push it plumb. Once it's good, toe nail it in place until you can properly brace it. Proper diagonal bracing is "let in" to each stud as it passes over them making the brace flush for the sheathing.
Sheathing alone will brace it, but it aughta be plumb first, then nailed. The 4 X 8 sheathing can be used as large square, better than a 24" framing square in the corners.