Was the box and torque converter completly drained as that's a lot of ATF if is wasn't.
Some spec discos have a switch that only allows you to move from (P) when the brake pedal is pressed. You don't have the transfer box in (N) do you?
This has been copied from Ashcroft's site.
I have just fitted a reconditioned ZF autobox. I have filled the box with the correct amount of oil but have no drive at all, forward or reverse. Why?
The first thing to do is to remove the oil cooler pipe from the bottom of the gearbox (put a container under the open end to catch the oil) and start the engine. (You should get a good flow of oil.) If not the pump has been broken during installation and will need replacing/rebuilding. The torque convertor must be fitted to the gearbox, located correctly on the pump and locked into postion before the gearbox is offered to the engine, otherwise damage will occur to the pump, resulting in no oil flow and subsequent to other parts of the autobox.