I'd say the first thing to do is to jack both back wheels off the ground and turn 1. It should turn freely and the other wheel turn in the other direction. The only 'resistance' should be the weight of turning the wheels. This will prove that your brakes are not binding.
Then you need to have just 1 rear wheel up and to a proper 1 wheel up test - see how long a weight takes to fall on a bar fixed to the wheel (hub nut). Compare this to the other 1 wheel up test results to see that the resistance is 'within normal range'.
If it is, then your 4WD system is working as it should - the diff and IRD gears are in place and the VCU is transmitting the correct levels of torque. Your rear wheels will be turning if the fronts are spinning.
As roccotune says - its possible that under engine load, if the bearings are worn in your IRD (or diff) it is possible that gears will slip - but it will be accompanied with an awful lot of noise!