Multiuser MIMO (MU-MIMO) is a new technology adopted in IEEE 802.11ac for enhancing downlink throughput. 802.11ac has an explicit feedback mechanism that enables very high quality channel feedback to be provided to the transmitter, but the overhead can be quite substantial. We propose a differential feedback method that significantly reduces this overhead by using a simple lossless, variable-length compression technique that exploits the slowly time varying wireless channel.