Overnight sagging is almost always a leaking valve. A member over on the Diesel Forum sells a rebuild kit for about $10 and somewhere on that same forum is a DIY for the rebuild. Do some searches over there and I'm sure you'll find what you need. A rebuild fixed my sagging rear end.

I would rebuild the valve and while the system was open, check the accumulators by removing the longitudinal fitting and inserting a small rod such as a pencil into the cavity. If it doesn't meet any resistance and goes all the way in, they are shot. If yours are original, they have almost certainly failed. I'm not sure what you mean by "bottoming hard", but failed accumulators usually result in a hard bouncy ride. New accumulators made my wagon ride like a different car.

In the hundreds of posts I've read from TD owners I have never heard of a pump failing and don't know if they are rebuildable. The actuators (hydraulic shocks) are also pretty robust and not prone to failure.

As far as height goes, guys usually measure to the side body moulding from the ground at the front and back to check for proper height. Your springs may be a little tired after all these years, but you could also change your height by adjusting the arm on the valve.

__________________
1981 300TD 310k miles
1970 280sel 172k miles
1966 230 Fintail 162k miles

"Where are we going? And why am I in this hand basket?"