This is the problem, exactly as GarryCol wrote.
A Chain reaction...
If your Viscous Coupling is stuck solid, you have to replace it soon as possible in order to keep the rest of transsmision healthy.
I was there few months ago with my 1.8K '01 model. Was really expensive but at least saved my IRD, rear diff, propshafts and Gearbox. (a lot of money).
You can test your VC to be sure: Jackup one rear wheel with 1st gear on and handbrake off. Then try to turn the wheel with both hands. For a healthy unit the wheel should turn very slowly with resistance. If the unit its solid the wheel wont move at all. Maybe just an inch.
Also check all the joints and propshafts from end to end, go under the body an try to move the propshafts with your hands, nothing sould move.
George