4WD Issues?

I was hoping somebody could help, I have the truck listed above and whenever I've been driving on the motorway and then pull off on the slip road, as I'm slowing down, it will momentarily lose drive and then switch itself into 4WD low range. I have to then pull over and actually get it to engage 4WD low range to get it to switch back into 2WD High. It can then take a few goes, going a few feet, back and forward and restarting the truck to actually engage it back into 2WD. I've noticed that whilst I'm driving, the green 4WD symbol will occasionally flash.
The question is, would this be a problem with the actuator or the transfer box itself, and is there any way of disabling the 4WD system completely until I can diagnose and repair the fault, ie removing the fuse from the fuse box under the bonnet, so it doesn't engage at all, for the time being, as I need to use it to get to work to actually afford to fix it!
Also, when I brought it, it had no service history so I'm not sure when any of the axle, transfer case or gearbox oil was actually changed.
Any help would be greatly received, and I will reply to ASAP.
TIA, Stuart