It is very common to crack there. The fix is to weld the crack and then make a gussett out of 1/4 flat steel. Shape it like a triange except on side is 1" and the other is 2". Put it in the top off the frame with the short side against the swingarm mounting flange. The long side running 2" down the tube toward the front. Weld it to the flange and down the frame. If you haven't already check your upper shock mount it usually cracks too. It needs the same steel layed flat and welded to the mount and to the frame and also both tubes where they meet.
Sold the quads so not as ....
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