I'm assuming the rear system is fed by the factory A/C compressor?
I have a 2005 mostly stock E-350 with the factory rear heat & A/C system----that had to go for a number of reasons, replaced with a Pro-Air over head mounted heater using the factory coolant lines. I had thought of using the various A/C block off devices but eventually opted to simply remove every bit of the factory A/C parts for the rear evaporator. This left the van with front-only A/C.
I dug into the van and parts drawings to determine the differences in the two systems eventually needing only a few parts: one Ford high pressure line, a new orifice tube and receiver/dryer for the front only systems.
This has worked out well, the existing front only A/C is as simple and as basic as you can hope for, cost was under $250 using Ford or Motorcraft. Naturally evacuating and recharging the system was additional cost, done by professionals who are also friends and customers.