Hi Adrik,
Unfortunately, Escalades are known for their bad transmissions. The problems with hard or difficult shifting is a known issue, and the problem is almost always related to the valve body.
If you are not familiar with automatic transmissions, the valve body is an arrangement of valves, check valves, and other components that control the flow of fluid through the transmission to shift gears by rearranging the relationships between the various planetary gears that give a transmision "ratios" like a manual transmission.
Automatic transmissions also have a clutch pack that holds the planetary gears in position while the transmission is in any given gear. These clutch packs are engaged and disengaged by hydraulic pressure from the pump in the transmission, but the actual circuits that control the movement of the clutch packs are controlled and regulated by small valves in the valve body.
With Escalades (and other GM transmisions), some of these valve sometimes stick halfway open or closed, which means that the clutch packs often do not fully release to allow the planetary gears to change position relative to each other, and it is this what you are hearing when you say you hear a scratching or grating sound. In effect, the transmission is still in gear (or neutral), but because the clutches are still partially engaged, it is often impossibe to move the selector lever without breaking something in the transmission.
The problem is that this kind of problem can appear suddenly. The transmision may be fine for years, but the next time you want to engage a gear manually, it does not work, just like you are describing. However, fixing the issue is not always as easy as replacing the fluid, since the valve that is sticking sometimes has to be freed by hand.
This is not always the case though, and a fluid replacement might fix the issue, but don't bank on it. If a fluid replacement does not fix the problem, you may have to replace the transmission because repairs are almost just as expensive as a replacement.
Moreover, if the technician repairing your transmission is not an expert, chances are excellent that you will end up experiencing other issues when you get your car back. The transmission could be noisy, not shift correctly, or not shift at all, so you may want to consider replacing it instead of repairing it.