Auto hubs not disengaging

Gday, I have an '86 td Jackaroo, which recently started occasionally making a grinding noise from the hubs when i reach about 40kph. The noise comes from either hub, but only one at a time, and goes away when i slow down. I took it to my mechanic, but he couldnt work it out as it doesnt do it all the time, and it wasnt playing up when i took it to him. All he could see was the hubs were dry, so he greased them, but they still do it. I looked it up on the net, and a couple of sites say this is the hubs not disengaging properly. I always back up a few metres after using 4x4 but the hubs still kick in when they shouldnt. Does anyone know why they might do this ( i live in the northern tablelands, could it be the ball freezing winter mornings locking them up?), and whether i could change over to free wheeling hubs and would this solve it. Cheers