Honestly, I love that you asked this question as I was wondering the same thing. I have a 6400 and that's on my "what am I going to do about that" list. I want to say I saw something about a conversion kit somewhere along the way. You can certainly get an electronic solenoid switch to make the change but I seem to recall that I read there is also a gearing change that has to be made with the speedo when you shift the rear end. The vacuum system does that for you but if you change, you have to make that speedo adjust manually somehow...
Disclaimer: NOT an expert at all but trying to recall what I read.
