I don't think there's any rumour of a V8 diesel (Jaguar doesn't have such an engine).
The probability is that the Range Rover will get the 2.7-litre ex-Jaguar V6 diesel used in the Discovery 3 / LR3/ Range Rover Sport sometime soon.
The current Td6 is a BMW engine from when BMW owned LR. The BMW engines were expensive for Ford to buy from Germany and spares were equally expensive. The new 4.2S/4.4 V8s are Jaguar/Ford engines, so it seems logical that the Range Rover will get a Ford-group diesel some time soon, along with the Defender.