The reason you can't find them is that they don't exist. All US military references used the commercial designations for these engines.
Why? Because (except for the YC-15) the airframes were purchased as Commercial Off-the-Shelf (COTS) and then modified for their mission. Since the engines were already integrated into the aircraft and had an existing logistics network (documentation, suppliers, et. al.) there was no need to replicate everything within the DoD system. To do so would have been a very big expense for no real added value.
There were only 48 C-9 and 19 T-43 aircraft purchased so economy of scale was very critical in keeping costs down. And the JT-8D wasn't used in any military unique airframe.
The KC-46 is planned for 175 aircraft, but it too was designated as a COTS airframe purchase. So the engines retain the PW4062 designation. While working at a Boeing supplier providing avionics for the C-46 we had to maintain all the Type Certificated functions of the B767 while adding the military capability.
As an aside, I've worked on several products in the past that were purchased as COTS by the military. Most kept their commercial designations. A few that were significantly modified did get military component designations.
It will be interesting to see if the engines selected for the B-52 re-engining are redesignated. They might, but the goals indicate they may retain commmercial designations. From Air Force Magazine (Jan 21, 2019):
Officially, the project is called the B-52 Commercial Engine
Replacement Program (CERP), and the Air Force has already hosted a
number of industry days to dicuss it at Barksdale AFB, La., most
recently in mid-November. The stated goal: Obtain a “commercial,
off-the-shelf, in-production engine,” according to a FedBizOpps
(Federal Business Opportunities) announcement regarding the industry
day.
The YC-15 was a prototype and would not have had any logistical infrastructure set up beyond that necessary to support the flight test. The selection of the JT-8D for the YC-15 was again likely due to the cost savings as Douglas had them in their system for DC-9 production.
