Actuating Lever Slippage

[Alec]

I have an 85 300TDT with 272k miles and the suspension is showing signs of maladjustment and/or old age. The fluid resevoir maintains its level so I am thinking that the fluid isnt leaking. I have not been able to get under the car to closely inspect the shocks for leaks.
The symptoms are as follows...when running the car maintains level without fail. When I park the car over night the rear sags a little especially the left side. As soon as I start the engine the car levels itself.
I have read of an actuating lever that can be adjusted but how can I determin if this is the problem. My concern is that since only one side is showing the most obvious signs that the shocks may be on their last legs. Any input of where to first look to isolate the problem would be very helpful.

Thanks for you time,
Alec.