not sure exactly when ford got stingy and started casting them, But they were definatley a different design. the cast beams have the ball joints pressed into the spindals, and on the forged ones they are pressed into the I-beam. I had an 89' f150 that had forged beams, and my friend has a 92' f150 with cast beams. so it was somewhere in that time frame if that helps.
GOD BLESS AMERICA! and hopefully my poor truck too.