As usual, dxb is right. As this Usage Note from AHD4 shows (http://www.bartleby.com/61/11/D0301100.html) both forms are presently accepted but the weak form, dived is the original, at least working from the OE verb, dyfan. The other verb mentioned, dufan was strong; the first preterite would have been deafan, however *that would have transmorgrified in MnE.