You can retrieve neighbor LQI information by sending ZDO Mgmt_Lqi_Req commands to devices on your network. You should be able to self-address this command to also recieve LQI information from your local XBee. These commands can be sent using API Frame 0x11 - Explicit Addressing ZigBee Command Frame. The payload of this ZDO command and its response can be found in the core ZigBee specification which you can register for and download from www.zigbee.org. You might also be able to find secondary documentation on that command online.
RSSI values can be read off of the XBee via the ATDB register. This provides the maximum RSSI during receipt of the most recent packet. See the XBee documentation for more details (how it translates to dBm, etc).