Wiki says the LLV's chassis is from a chevy blazer. The engine is also from GM. The rest of it is modifications needed for its role.

The original LLV was designed as a custom body atop the Chevrolet S-10, including using the original drivetrain. Everything aside from the body comes from the S-10, hence why they were able to keep them on the road for so long.

Even as simple as it was, the LLV was a major departure from the Jeep DJ-S, which was just a modified right hand drive Jeep CJ-5 with a sliding side door. In both cases the off the shelf parts available directly from the manufacturers extended the service of first two official USPS mail trucks well beyond their intended lives. Something which is unlikely with Oshkosh given the heavily custom nature of it. Refits to the LLV have included buying drop-in replacement LED headlights to replace the original square sealed beams since the form factor is a standardized part shared across the entire automotive industry from the 1970s onwards. Meanwhile the headlight assemblies for the NGDV are custom made, from the shrouds to the LED driver boards (an expensive problem shared with most headlights these days). We may have a similar problem to the Cadillac XLR's taillights where the driver boards make it infeasible to replace them.

The USPS has a bunch of highly-specific requirements that they aren't willing to change, which makes an off-the-shelf van impossible.

These are things like: giant unpainted bumpers, sliding drivers door, the rear door has to be roll-up rather than normal doors, step through from front to rear, some size requirements.

One of the alternatives which was rejected when they signed the contract for the new vans was a customised Ford Transit.

Might be wise to rethink those instead of trying to be unique. Haven't heard about "custom post vehicle" anywhere else in the world.