I had a 99 Expy that I was trying to disable the AWD. I did run without the t-case hooked up, and I noticed alittle gas mileage difference. The problem is what everyone has said, the front diff stays connected. I even researched Ford to see if I could buy the vacuum pod that disconnects the front axle. The internals of the front diff do not have the parts needed to disconnect. So you'd have to buy a front axle from an F150 or a expy that had the 2hi option, then for everything to work right, there are 2 solonids on the firewall that control the connect/disconnect of the front axle. Then to control all of that you could do your own switch, or get a GEM from an expy that had the 2hi option. I'm not sure though if the t-case is different. You may still be able to use the one you got, but have the plug un-hooked, this is the part I'm not sure about because I traded it in on the 00 harley F150.
I can't stress enough though, leaving the front diff locked without the t-case plugged in makes the handling weird, like in the rain, the rear will slide around, but to counter steer with a locked front diff, well lets say its a bit scary. If you persue this, let us know how you make out. To help too, there are a few expy's made between 99-02 that did indeed have the 2hi option. I test drove one and it was a 00 e.b. and it had a 2hi on the switch. I asked a ford tech about this topic and he said he'd call me back with info.....that was almost a year ago!