4x4 issue with grind/hum noise

Hi Guys and Girls,  I've got an issue with my 4 wheel drive not working and alarmingly a noise coming from what I believe to be the transfer box but I could be wrong.  What I know is... There is a humm/grind above 28mph and severe between 28 and 40, it's intermittent. A garage has had it on the ramp and says the 4 wheel drive system isn't engaging and that the front prop doesn't spin, however when you press the buttons I can hear the motor in the transfer box doing something and all sounds positive.  The Jimny is a 60 plate with 102,000 miles on it and it seems to have been looked after.  The garage I took it to believes that it is trying to put itself into 4 wheel drive low as your driving but this doesn't make sense to me (I'm in engineering but by no means a mechanic or have any good electrical knowledge). This garage recommended I check some solenoids and failing that change the transfer box which I think personally may solve the problem as it seems an obvious place for the problem.  I'm told by another garage that the rear prop had seized and this was replaced at dealers expense. He also said the hum might have been the tyres or a wheel bearing, but I'm certain that isn't the case.  Does anyone have any ideas of what to look at?  Thanks in advance