Strange problem

Hi all. I have a strange problem on my disco 300tdi. It's a manual gearbox. When I reverse if I push the clutch down and roll it tries to lock up. It's as if the brake is being jabbed on and off. I've checked the handbrake and its not binding. I checked the transfer box oil level and it was almost empty so I filled it but the problem is still there. Any ideas?

You're sure it's not the hand brake? What happens after you've reversed under power for a while, or if you leave car parked without using the hand brake?

It's deffo not the handbrake. I've wound it right off and no change. I have just lifted the rear off the floor and spun a wheel and the opposite wheel turne the opposite way I then lifted the front and both wheels can be turned without the other turning which points to a front diff problem I'm thinking