cse1,
Diagonal cracks can be related to settlement of the foundation, however they are most commonly the the result of differential downward movement of the wall - or more precisely the downward movement of the floor on which the wall is set. Let the crack orientation direct you to the forces acting on the wall - remember Mohr's circle !!
In buildings with crawl spaces, I often detect excessive atmospheric humidity and moisture in the timber beams and floor joists. Excessive moisture causes excessive creep and also results in favorable environment for fungal decay, which of course weakens wood and causes it to deflect (or crush) more.
The deflection of the joists or timbers can be determined by pulling a few stringlines along the length of these members. Crushing at a support is fairly obvious - but don't confuse crushing for an intentionally notched end. Determine the size and spacing of the joists - is the floor system satisfactory?
From up top, a simple "jump up and down" should enable you to gage just how stiff the floors are.
The humidity in the crawl space should not be above 55% and the moisture content of the wood joists/beams in the vicinity of 15% max. You can probe the joists/beams with an awl, meat tenderizer or other sharp/pointy metal object - hard wood is obviously stronger than soft, punky wood.
If you detect rot or insect infestation, excessive deflections or crushing call a structural engineer.