Jeep Grand Cherokee WK CRDs have an issue with trapping dirt, debris and salt between the steel tank and the steel armour. The problem seems most pronounced in the rear left corner. At the very least try and flush this corner out when washing.
These two threads on Jeepforum explain the problem and solutions quite well. Below is a PDF I made that better explains how to remove the armour and some of the issues with doing this and fixing the problem.
The drain holes in the armour are small, and the tank seems to suffer. Attached is a link for a PDF with detailed instructions on how to drop the tank with photos showing where the bolts are and how to reach them. If you have a CRD (or possible a WK with a steel gas tank and armour) it is worth inspecting before you start having a diesel leak. With the amount of people needing this problem to be fixed, it probably should have been a Jeep Chrysler recall, but we know how loathe car companies are to recall vehicles and pay money to fix things they screwed up, unless you are Tesla.
Please feel free to submit a claim to NHTSA.
Basically if you have a Jeep WK with the fuel tank armour and live in an environment that uses salt on the roads there is a good chance your tank has begun corroding heavily, and many people have already had to fix this. Jeep DIesel Fuel Tank Armor Drop