In this paper, we apply q-ary LDPC codes to differential modulation systems, and study the design and performance of the resultant coded modulation systems. A low-complexity joint detection/decoding method for noncoherent demodulation is proposed, in which the hard-message-passing strategy is used for a joint factor graph. It combines trellis-based differential detection aided with channel prediction and the reliability-based decoding of nonbinary LDPC codes introduced in [1]. The Max-Log-MAP algorithm with soft-in hard-out is used for the differential detection. Simulation results show that the proposed method can offer good performances with a greatly reduced complexity.