Dual-function radar communication (DFRC) systems implement both sensing and communication using the same hardware. An emerging DFRC strategy embeds transmission of digital messages into agility-based radar schemes in the form of index modulation (IM). This approach provides the ability to communicate without entailing degradation in radar performance, at the cost of increased decoding complexity at the receiver side. In this work we propose schemes for reducing the decoding complexity associated with IM-based DFRC systems. We first focus on the receiver side, developing a sub-optimal low complexity scheme for recovering IM symbols embedded in radar waveforms. Then, we propose a method to modify the radar waveform to facilitate the recovery of the communicated bits with minimal effect on the radar performance. Our numerical results demonstrate that the proposed techniques allow the receiver to reliably recover the transmitted symbols with an affordable computational burden.