Crabbing

Or , u could just tape measure from the trans. crossmember to the rear axle shaft, compare both sides , no need to make it harder than nessesary .

Yes, you can, but that eliminates the frame rails, doesn't it? DOing it that way and you don't know "where" the discrepancy might be. .....but, doing it both ways will tell you for sure. Because since the rear axle bolts in, there could be some differences there apart from the frame.