Emulsion & DeCarbon Shock Absorbers
Jason,
The shocks really don't go bad. The change you are experiencing at this mileage is probably more likely your entire suspension just sort of settling in. The bushings which are at all of the attachment points between the major components are going to take a set and eventually wear out completely sooner of later depending on what you do with the vehicle.
As for the shock absorber, which is really not the right word for them, about the only thing that can go wrong with them is the oil leaking out of the shaft seal. One other thing can be the plastic gasbag which is between the outer external tube and the inside tube. It can get hot and melt, which can let the gas into the oil. A good bit of heat is needed to cause this that is what shock absorbers really are for, making heat out of mechanical energy. The air springs are what absorb the shock from road irregularity. The shock absorber is for damping the movement between the chassis and the axel. If you manage to get the shocks hot enough to exceed there duty cycle then the heat in the shock gets to a point where the chrome on the shaft will turn blue up by the shaft seal and you will literally destroy the piston rod shaft seal in the end of the shock body. When the shock or shocks have cooled down from the driving which caused this extreme heat load, the seal will begin to leak and during rebound damping cycles the piston will try to force the oil out of the shock bodies internals. This reference is for an emulsion type shock in the above case.
Now if you upgraded to some Bilstein's you would find that the construction inside the shock body uses a floating piston which is at the end of the shocks body riding inside the same 46mm diameter tube in which the actual piston is mounted to the rod which extends out through the open end of the tube were the piston rod seal is located. This floating piston has ~300psi of nitrogen gas behind it. This pressure exerts a force of 772 lbs on the oil inside the tube were the oil is located when it is at ambient conditions. This is why in part the single tube De-Carbon shock design is so much more superior to an emulsion tube shocks gasbag. The oil is pressurized so that when a harsh jolt is sent into the suspension the oil doesn't cavitate as it is being forced past the disc spring valving on the piston. However, if you cook these shocks the 300psi will make a quick job of expending the oil until the floating piston is riding on top of the actual piston and rod assembly. (Thank you Edelbrock for that experience. Edelbrock, your workmanship standard stinks on the IAS performer shocks. But that is another story!).
I hope that has helped your understanding and perhaps might sway your decision when it is time to enhance the performance of your RR.
