3G wireless data networks have emerged as the first high speed, ubiquitous data networks. 4G wireless networks are poised to replace 3G networks as the next generation of mobile data networks, but the transition will be lengthy and expensive. Wireless authentication across 3G and 4G data networks is typically an arduous task, requiring complete disconnection from the existing network before performing a lengthy full authentication on the new network. We introduce an efficient interworking method for CDMA2000 1xEV-DO and WiMAX data networks that allows existing authentication credentials to be leveraged in more efficient interworking protocols at both tightly- and loosely-coupled network integration levels, as well as a proactive handoff scheme that takes advantage of our tightly-coupled interworking. We perform a detailed simulation and mathematical analysis of our interworking and handoff protocols and find that our interworking scheme reduces interworking delays by up to 75% in the WiMAX to CDMA case and 85% in the CDMA to WiMAX case.