We propose and evaluate a scalable position-publish and an accompanying routing protocol which is efficient despite operating with imperfect information regarding the destination's location. The traffic generated by our position-publish protocol fits within the transport capacity of large mobile ad hoc networks (MANETs) with constant communication bandwidth allocated for routing overhead, even as the network size increases. The routing protocol guarantees, with high probability, routes whose lengths are within a constant "stretch" factor of the shortest path from source to destination. The key idea underlying the scalability of the publish protocol is for each potential destination node to send location updates (with frequency decaying with distance) to a subset of network nodes, structured as annular regions around it (the natural approach of updating circular regions in distance-dependent fashion does not scale). The routing protocol must therefore account for the fact that the source and/or relay nodes may not have estimates of the destination's location (or may have stale estimates). Spatial and temporal scaling of protocol parameters are chosen so as to guarantee scalability, route reliability and route stretch, and these analytical design prescriptions are verified using simulations.