Hi Tim, welcome to the best Landrover site ,I ve had mine for 9 mths and learnt more on here than from a workshop manual .
By heavy load what do you mean internal or a trailer?
My own 2001 TD5 Auto with SLS is dependent on how much weight is on the back axle as to how much the back end comes down. There is a certain amount of compression but as long as you don't see the top of the back tyres touching the arches you shouldnt really have a problem, best way is to load up and spray soapy water on all connections including the bags remember to check the compressor connections as well ( Under the passenger sill ).
As for the off road mode it moves about as quick as a glacier on mine but eventually get s there. Try closing the drivers door and function the button then physically put a tape on it as it is quite hard to see it moving.
Good luck
Ciaran