Do you know what was changed? Do you go off road at all? Longer shocks can increase suspension flex, you don't need to have the lift to get this benefit. But there are many many different ways to modify the suspension.
Bang sounds more like a half shaft. What sort of noise? When you say the diff was noisy?
There are like 100 rotating things between the drivers seat and the road. Very unlikely to be able to isolate and say a diff is noisy while driving.
Not...
This is my build here: https://www.landyzone.co.uk/land-rover/200tdi-s3-88-pas-long-travel-suspension-rebuild.298104/
Having also got another 200Tdi powered Series, I'd also suggest moving the alternator to the other side of the engine. Gwyn...
As it stands, £200-500 really, but nobody is likely to accept that. The chassis and engine are worth money on their own, but obviously require work to get into individual bits. As a rule, parts will often add up to more than an entire vehicle.
A...
Do you know what the exact failure was of the previous two diffs and what bits where actually replaced?
The front axle has many components, the diff is not normally a wear item and only really fails or breaks under duress of extreme off roading...