Prado 120 out of sink

Did a trip to Robe last week and then up to Mildura and then back home to near Geelong.
While at Robe we did some beach driving and while doing so noticed the back wheels of our Prado driving quicker than the front wheels. Because of this I couldn't keep the back wheels in the same track as the front wheels and needless to say, because of this we got bogged a few times!
My brother in law's got an older discovery with automatic (mine's a manual) and he had no trouble going through the sand. I had to go flat out in low second and he just crawled along.
I know automatics are a great advantage in sand but i still think my main problem was crabbing through the sand. I've also noticed it doing this in mud on the farm, when we use to have rain!
On passed trips this hasn't been as noticeable when pulling a trak shak?
Having spoken to a mechanic at Mildura he asked me what sort of tyres I had and I said slightly worn out grandtracks. And his comment was 'there's your answer, with those useless tyres you track everywhere.'
Is this the answer? Or have i got a problem with the syncronisation of front and back wheels driving?
Brother in law who works for the DSE said he found similar problems with two late model prado's in mud.