I have a CUCV, a 1984 M1009 Blazer.
Mine originally came from a base in California, but went to my local Nat. Guard base before being sold at auction.
Stands for Commercial Utility Cargo Vehicle.
The M1009 was the Blazer, but it really wasn't a "K5" anymore, because they beefed up the springs and other components, so it's really a K10 blazer. The M1008 and M1028, are single rear wheel K30s (one with bed, one chassis model) also beefed up to 5 quarter ton status. The M1031 are dually K30s.
All came stock with the 378ci - 6.2 J-code Detroit Diesel, a TH400 transmission, and NP205 transfer case. No manuals.
They were made from I believe late 83 to 87 when they stopped production of squares. Chevy then started making the CUCV 2 from 88 to 98, which was based on the next gen chevy truck. Didnt have the blazer back until 92 when they came out. Had the same driveline as previous CUCVs until 95 when Chevy dropped the 6.2 and upgraded to the 6.5 duramax Turbo diesel.
Before 83, they used dodge W200/W300 called M880 and M890s, that had a mopar 318 and 727 auto.
In 88, they also started production of the first M998 HMMWV (aka Humvee), which had the same 6.2/TH400/NP205 driveline. In 95 they also got the upgrade, and got the 6.5 turbo diesel and Allison trans.
But bases everywhere would buy civilian vehicles as take home trucks and run around for army reserves and big wigs.
Your Jimmy was probably bought because of the need for the K5 size truck, and because the guys in the Motorpool were familiar with squarebodys and had plenty of parts trucks.
Just showing off my bit of trivia I know about CUCVs and other Military trucks, sorry.
Don't ask about military Willys, we'll be here all day, lol.