What parts, if any, are interchangeable between the Jeep Cj/Wrangler, and the Postal Jeep…technically, the DJ-5 made by AM General, then the military contract division of AMC.
IIRC the first Postal Jeep was the DJ3A. It was a two-wheel drive model with a two-speed automatic transmission. I believe the non-Postal version had the regular three-speed standard transmission in addition to having left-hand drive. The DJ-5 replaced the DJ3A. See The CJ3B Page for more information. (Link is to just one of the over-700 pages on the site. The CJ3B Page homepage.)
One good thing about the old Willys/Kaiser/AMC Jeep is that many of the parts are interchangable.
Oh. As far as I can remember, parts were generally interchangable from the CJ2A through the CJ-5. For example, I think a tailgate from a CJ2A would fit on the CJ-5. A lot of the mechanical parts were common as well. Up through the Kaiser-Willys CJ-5 the civilian jeeps were called ‘Universal Jeeps’ (or sometimes ‘Jeep Universal’). There were engine upgrades between the MB and the CJ-5 Universal, and I think more changes were made when AMC took over. I think it’s less likely that CJ-7 parts would fit as neatly on earlier models than CJ-5 parts would. While the YJ and TJ Wranglers retained the basic ‘spirit’ of the classic Willys, they are much different machines.
Nothing much to add, except you can try the CJ forum at JeepsUnlimited.com. Lots of knowledgeable Jeepers frequent JU.
I created The CJ2A Page, only I gave it to one of my readers last year because… Well, you all know the drama I was going through last year. I just couldn’t keep it up anymore.
Anyway, the new guy has put it on a much better server and he’s added fora much like the ones here. Go on and check it out. 
I was told once that the PO used the little jeep because in the event of war it could be converted to 4x4 and used by the army.
Any truth to that?
I don’t think so. If you check out the old magazine ads on The CJ2A Page, you can see what a useful little vehicle the Universal Jeep was. You could use it to plough your field during the week, and drive it to church on Sunday. In a way, it was sort of the Unimog of its day. But not everyone needed 4WD. Why pay for it if you’re never going to use it? So Willys made a ‘Delivery Jeep’ – the DJ. As the name implies, it was meant to be a light delivery vehicle. Four wheel drive wasn’t necessary in town, and the big box and the sliding doors made it very handy for its job. It was a natural for mail delivery. And there were a number of Jeeps made with right-hand drive. It was easy to put RHD on the DJ so it could be used by Postal carriers. So what you had was a rugged, inexpensive, reliable vehicle that was perfect for deliveries and had mountains of spare parts.
Was conversion a possible reason for choosing the Jeep? Maybe, but I’ve never heard of it. There were thousands and thousands of CJ2As, CJ3As, CJ3Bs, M38s, M38As, and CJ-5s running around. Not to mention the plethora of MBs and Ford GPWs (‘Willys Pattern Jeeps’). So it doesn’t seem likely to me that DJ-toCJ conversions would have been a consideration.