Feels like a rear end shunt on hill start

Any clues as to why when pulling away from a hill start with the hand brake there is a bump which feels like the landy has been rear ended by another vehicle.

Any advice on what could be the problem and how to check would be great.

Does it happen when you let your foot off the accelerator when traveling at speed? Clunking/bumping at the rear could be the a-frame ball joint.

Not noticed it at high speed and the landy is noisy to say the least, the bump on the hill start is quite severe.

Checked your propshafts are in good order? Also might be worth checking the handbrake is rotating freely.

Is your hand brake releasing smoothly or is it binding? Try facing down hill and slowly / gently release the handbrake (in neutral), does it release and roll forward smoothly and can you control the speed by gently increasing and decreasing the handbrake pressure? If it suddenly releases it may be binding so the clutch is fighting the hand brake and when the hand brake releases you get a lurch forward.

Ours did this and it was the handbrake binding due to oil leaking into the drum from the transfer box.
You can test for this by following the advice given in the post above.