Unfortunately your problem is pointing to an issue in pack 7. Gears 1,2,3 get drive from pack 6 which is a nice big pack, unfortunately pack 7 is allot smaller and tends to wear out more quickly, particularly if loaded up in 4th gear too often. Either a pressure issue in the piston seals or the pack is worn out. I have seen other dramas that lead to a drop in supply pressure to pack 7, none of which are addressed with the box in. Have a look at your pump pickup screen at the back to confirm some brake material or mechanical failure. Sometimes failure somewhere along the main shaft can lead to seal misalignment on the end of the shaft that carries the oil to pack 7. Either way early intervention is the key. Don't forget to replace the difflock solenoid when the box is out, it is a ***** to get too. Hope I have helped, all issues I have encountered myself. catgraderhire.