(T) Suspension dropped

Hi all its been awhile since I last posted as all is or was going well with my 2005 1.9 jtd cargo van. Recently a friend said while driving behind me the van was leaning to one side the drivers side to be precise and while it was parked up on fairly level ground with hardly any weight in the back it does seem that the rear drivers side is lower than the passenger side, I can get 4 fingers under the drivers side just about whilst the passenger side 6 fingers easily, van has done 140,000 and serviced every year since new and new mot in April 2017, just wondering wether anyone else has had same problem and would it be the leaf springs or the shock as I can't see any noticeable difference from underneath whilst I lie on floor.. any help or advice always appreciated. Thanks in advance

Bottom of the rear shock goes rusty and leaks fluid,have a look if it's dry or oily.

It is dry but does have some rust on it going to have a mechanic take a look at it later today thanks for your reply greatly appreciated. If it was the leaf springs then would it be best to change the two at same time save me another job if the passenger side goes in the near future?

I would say yes,if one's gone the other won't be far behind and it's much easier and cost effective to do the job in one hit than to do the same job twice,also where load bearing/suspension parts are concerned it's best to replace both sides at the same time to keep things in balance.

Are the conical sponge spring assisters in tact they are very important and cheap and easy to replace they do ware away over time?

Very disintegrated on both sides so will be asking my mechanic to change them same time when he has a look at it laid up atm so the van and me are off the road thanks all for your help and advice