Multiuser diversity gain is an effective technique for improving the performance of wireless networks. This gain can be exploited by scheduling the users with the best current channel conditions. However, this kind of scheduling requires that the base station (or access point) knows some kind of channel quality indicator (CQI) information for every user in the system. When the wireless link lacks channel reciprocity, each user must feed back this CQI information to the base station. The required feedback load makes exploiting multiuser diversity extremely difficult when the number of users n becomes large. To alleviate this problem, this paper considers a contention-based CQI feedback, where only users whose channel gains are larger than a threshold are allowed to transmit their CQI information through a spread-spectrum-based contention channel. Considering the capture effect in this contention channel, it is shown that: 1) the multiuser diversity gain can be exploited regardless of the number of transmit antennas at the base station; and 2) the total system throughput exponentially approaches that of the full feedback scheme as the spreading code length of the contention channel linearly increases. In addition, it is also shown that multiuser diversity can be maintained with feedback delay of time-variant channels. We also consider the issue of differentiated rate scheduling, in which the base station gives different rates to different subsets of mobiles. In this scenario, mobiles feed back their CQI with some access probability, and we show this technique causes only a negligible throughput loss compared with the case without supporting differentiated rate.