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. 
http://www.jeepforum.com/forum/f67/rusted-out-crd-tank-3106977/
http://www.jeepforum.com/forum/f67/2008-crd-fuel-tank-removal-3423698/
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.
https://www-odi.nhtsa.dot.gov/VehicleComplaint/
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
Thanks for the information.
I’m new to the WK , having bought an 08 CRD with 93,00 kms.
I may have purchased a money pit (hah).
The differential ratios (3.73) may be fine for pulling a trailer, but not so good for efficiency.
I’ve changed both diffs to 3.07 ratio (used front diff and used rear gear set), so that should help.
Then I noticed very notchy u-joints in that very stupid composite rear driveshaft. What an engineering failure that design is.
Now, since I live in the rust belt of Western Canada, I see that fuel tank corrosion is an issue.
What the heck? Every thing that I’ve owned since 1995 has had plastic fuel tanks. And this thing has to have steel? I don’t understand how design decisions are made by the manufacturers.
Well, at least through the forums (once a person gleans out the non-info submitted by many) I can see what the issues are.
Thanks for keeping WK owners in the know!
JP
Yeah I am not really sure why they chose to use a steel tank instead of plastic. Let me know if you find out.
Fuel tank. Finally, I tackled the fuel tank corrosion issue.
The worst part of this remval was trying to get the electrical connector apart. No amount of juggling, pulling, twisting or cussing would get this connector apart. I don’t know if road grime/salt was cementing the 2 section together or what. I had to kind of break it apart with a small screwdriver. Yes the lock tab had been removed from the start.
The small gravel and rust were in the left rear corner as described. No where near rusting through though.
Wire brushed tank and armor and coated rust areas with rust converter paint. Then used roof tar with fiber to coat the bottom half of the tank. This material had to be applied with a small trowel. The interior of the armor plate was coated with roofing tar applied with a brush. These 2 coatings will “skim over” but will remain very pliable with excellent adhesion. Placed the tank into the armor bucket. Then I had a good look at the exposed opening at he left rear area of this combination. AHA! Keeping the rocks out would be the secret to a long repair. I used expanding foam spray to seal all around the armor bucket to fuel tank opening . This will keep the small rocks and probably most of the liquid from entering the armor bucket and getting under the fuel tank in the first place. Water proofing the “seam” is not the primary function of the foam. The primary function is to keep the small rocks out to prevent the fuel tank paint/coating from being damaged and allowing rust. The foam will last since there is no possible exposure to sunlight. The 2 tar coatings will merge at whatever points they contact at the bottom of the tank. Only water might accumulate at the bottom but will dry out during driving. Should be a rust free area for a long time. Disassembly, if ever required might be a little messy. Foam cut with a keyhole saw blade and maybe a little sticking of the fuel tank to the armor bucket. This cost me approximately $45 Cnd for the small pot of fiber tar, the gallon of roofing repair tar, and a couple of disposable brushes. Oh, and lots of Corrosion X on the tank electrical connector and a little bit of silicon to make sure the connector stays “connected”. I sometimes wonder if some of these automotive harness connectors are a “one time use only” design! Work well during initial assembly, but disassembly is optional, or not feasible. Thanks for the opportunity to describe my preventative repair. JP Calgary, Alberta
Glad you appreciated the info. It is not a small job, and there is lots of opportunity to mess up, but it is rewarding to know you can save your tank or guard, or both.