Yesterday I took Scott's advise and put a Sears P1 Die-hard in my CRD. $208.99 plus tax carry out.
http://www.sears.com/shc/s/p_10153_1260 ... 000P?mv=rrThe first thing I noticed was that the speed of the starter motor was unbelievable
I bought this jeep new and it has never cranked any way near this fast!! A year ago I had replaced the original red top with a new one with no noticable change
Now for the real test, like the original poster 50f degrees and lower seems to be the when my crd started having delayed hard starting and rough running. This has been since new but since it sleeps in a heated garage it was no big deal just an annoyance. Last night it got down to 35 degrees, not frigid but well below 50 and I left the crd sit out all night.
This morning I checked the voltage in the Optima Red Top that is now in my wife's 2000 XJ and it was 12.4 in a 59 degree garage. The voltage on the Sears P1 sitting outside at 35f was 12.7 volts
Not a lot but the difference in the CRD was night and day

The starter spun the engine noticeably faster then ever and the engine fired up like it was 80 degrees
I added a strip of Valcro so I was able to put the blanket back on the larger battery but it now has about a 3" gap
So for those with slow starting problems when cold this is definetly the first thing to check.
I suspect 50 degrees is when "After Glow" is initiated and the added current draw is the straw that breaks the camel's back voltage wise causing the ecm not to turn on as ATXJ stated above.
For the record I have the original fuel filter head with no leaks and an in tank lift pump that definetly cured my air in fuel problems. I believe the original filter head was a victim of air collecting in the highest point in the system and not the cause

And don't forget the filter head is mounted higher in the 06 model year to clear the bigger ABS/ESP pp.