Hi All,
As the title states I find that my diff moves back on one side, specifically the driver side.  I took some measurements which indicated that the spring pack on that side had sagged and was sitting 30mm lower on that side. With the springs being so old and knowing that they had been retempered at least once , I bit the bullet and bought a new set of springs and u bolts which I fitted today. I have fitted poly pads for where the diff locator plates sit and tightened them up to the limit of my strength but after gong for a short drive this arvo I found that the diff had moved rearward again on the driver side. I had a panel beater take some measurements of the car and he reckoned it was square.  Any thoughts on what could be causing it to move??
 
Cheers, Len

Only way the diff could move is it has come of the peg on the springs due to incorrect alignment and or tightening or the front bush has collapsed or the front spring hanger has come loose or the spring has collapsed.
New springs are unlikely to fail in the same way as old ones Id suggest looking at the other items. Did you use new bushes?
 
JP