Hi,
Do you mean where the driveshaft comes out of the drop box ?
If so, you have to dismantle the UJ in place so that you can get the split pin out and undo the castle nut holding the shaft in. You may get the nut undone without dismantling the joint but it is very awkward. Drain the oil and then once the shaft is off, replace the 2 seals and the sealing washer in the centre of the shaft.
Mike