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This paper presents a reduced-complexity soft-output trellis/tree equalizer for a high-order modulated MIMO system undergoing equal-power, Rayleigh frequency-selective fading. The algorithm reduces the complexity of trellis decoding by applying the M-algorithm twice, once to reduce the number of states in the trellis, and the other to reduce the number of tree branches emanating from each state. For soft-information, the algorithm utilizes not only those fully-extended paths reaching the end of the trellis, but also paths that are traversed and discarded in the pruned trellis. Our results demonstrated that the proposed algorithm is capable of achieving near-optimal performance with a much reduced complexity.