A $2\times56$ Gb/s 0.78-pJ/b four pulse-amplitude modulation (PAM-4) single-ended multiple-input multiple-output (MIMO) crosstalk cancellation and signal reutilization (XTCR) receiver (RX) is investigated for medium-reach (MR) backplane communications. An XTCR scheme based on active crosstalk extraction (A-XTCR) is proposed to improve the signal reutilization efficiency of the RX. By adopting the proposed asymmetric-inductor current-mode logic (CML) single-ended-to-differential (S2D) converter, the pseudo-differential output mismatch is alleviated. Fabricated in 28-nm CMOS, the RX achieves a bit error rate (BER) of $< 1\text{e}\,\,-10$ at 56-Gb/s PAM-4 transmission with a channel of 24-dB Nyquist loss. Compared with the XTCR based on the conventional passive crosstalk extraction (P-XTCR), the proposed A-XTCR technique has improved vertical and horizontal eye openings of the RX by 24% and 15% at 56-Gb/s PAM-4, respectively. The result corresponds to the best energy efficiency of 0.78 pJ/b/lane, which compares favorably with the state of the art.