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Multiuser and cooperative diversity gains have been studied extensively in recent years to improve communication reliability and throughput of wireless networks. Multiuser diversity gains have been exploited by scheduling users with the best channel to transmit in each time slot while cooperative diversity gains have been exploited by having users relay each other's messages to the destination. Most works in the literature consider these advantages separately and in systems that rely on a central scheduler. In this work, we show that the throughput of distributed random access systems can be improved by exploiting both cooperative and multiuser diversity where the transmission probabilities of users are adjusted according to their local channel state information. The fully-loaded stability region is derived as the performance measure of our proposed transmission strategy. The optimal transmission control that maximizes the fully-loaded region is shown to be a threshold-like function. That is, each user shall transmit only when its effective channel is sufficiently reliable. The effective channel in this case is defined as each user's contribution to the weighted sum throughput of the network.