maybe not that strange?
Is that front wheels and back axle driven? in that case, not so strange I think that that axle is digging as the front rear axle churns up the sand, which makes for a bad start of the back axle.
Anyway, I wouldn't be to concerned about the few 100miles of sand driving when contemplating trans africa. Most of the time you'll be on corrugated pistes, where I'd asume that more axles= more confort. otherwise it'll be tar or sand, mud huge holes,... , and you'll get trough everything, maybe get stuck so once in while, but a bit later you'll be unstuck. It's not a race right?