The problem with a 4x4 is that all the transmission vibration comes through the floor.
Start with the propshafts, check the UJs for wear/collapse.
Check the props aren't bent by spinning them slowly.
Check for balance weights that have come loose from the props or loose/missing bolts in the flanges.
Check the wheels.tyres for missing weights/nuts etc, mud inside the rims and any strange inflation, bulges etc in the sidewalls.
If nothing obvious try removing one propshaft and running in difflock, then try the other and see if you can identify which end of the car is at fault.
That should keep you busy, more importantly I hope this identifies the problem, otherwise it's going to get oily/messy/expensive
