Awful smell from drivetrain

Hi all. This has happened a couple of times now with my 2007 FL2 TD4 when reversing in rough or slippery terrain (forest roads and snowy roads). When reversing from a standstill, an awful smell possibly from PTU, rear diff or Haldex comes inside the cabin. I believe this is linked to the AWD system activating when one of the wheels slip. Any idea where the smell might come from?

I've noticed a small oil leak from the PTU but nothing on rear diff or Haldex unit. Haven't diagnosed the leak properly yet, but I might have to soon...