2010 Tribeca 4th to 3rd Clunk

[Sydney]

Started with one major clunk and had trouble moving. Mechanic got me to buy seconds diff, he changed it no difference. Then he got me to buy a seconds gearbox, he changed it and then the 4th to 3rd change clunked. Took to trans specialist, they just want to recondition at thousands of dollars but ran it through there computer and they said faulty gearbox. Supplier supplied another seconds gearbox and mechanic installed that. Once again the same problem 4th to 3rd Clunk. My mechanic said to buy a second Automatic Tans module which I did and he replaced that and reprogrammed it. No change, still 4th to 3rd Clunk. Took it to another different trans specialist and he just wanted to recondition it for thousands of dollars but did reprogram it to clear all the warning lights off the dash only. Said it's a faulty gearbox. Still had the 4th to 3rd Clunk and all the warning lights soon came back. Thought this to be impossible so this supplier supplied a replacement gearbox also. Mechanic changed to this third gearbox now and still the same problem, 4th to 3rd Clunk. My mechanic has just about given up and can't solve it, either has two trans specialists. Does anyone in the world know exactly what the problem is please? Can't be three replacement seconds gearboxes with the same problem. Desperate.