Hello everyone and thanks in advance for your help.

I have a 2000 f150. It is a 5.4L Mechanical shift on the fly 4x4 7700 series truck. It's pretty much all base package and it used to run on natural gas as well as normal gas although I removed the CNG tank years ago and have been running it in normal mode. The fuel pump died about a month ago and I slid the bed up and back to replace it. When I finished, the truck started up fine but the radio no longer turned on at all. I didn't look into it immediately since it seemed to be running fine otherwise. But then the snow fell and I realized that it wouldn't shift into 4x4 either. No indicator lights on the dash and no voltage at the vacuum solenoids (at first the 2x4 seemed to have 12V but I rechecked it tonight and found no voltage on either solenoid with the truck running) Both solenoids have vacuum in the lower hoses (I only checked with my finger tips). I bought a used GEM and swapped it out last weekend. When I reconnected everything and fired it up, the radio turned on and when I shifted it into 4high, the indicator light worked on the dash, but the heating fans wouldn't come on. I thought, OK, maybe I just need to get it reprogrammed to match my VIN but when I tried to drive it up my steep snowy driveway, it clearly was not in 4x4. I was out of time so I parked it and went back out to work on it tonight. When I turned it on tonight, the radio wouldn't come on again and the 4x4 indicator light wouldn't work as well when I tried shifting into 4high. It's behaving just like it was before I swapped the GEM except that the heating fans still don't work. Out of curiosity, I disconnected the battery for a few moments to see if it would reset the GEM and act like it did this weekend but it made no changes.

All of the fuses are good although I have not checked to see if they have power while running. I have a wiring diagram (that might be correct) that points me toward fuse 6 (5A) and fuse 23(10A) in the central junction box (left side of dash) and fuse 104 (30A) in the battery junction box. This diagram also states that there is a 'shift on the fly relay (in RPO relay block). I am not sure what that is or where to find it. The relay looks like it shares a common ground with the solenoids on fuse 23? and it sends a signal to the magnetic clutch that is in the transfer case assembly. I am not very good at reading these diagrams so please correct any mistakes.

I have been reading this forum, trying to make sense of the system so that I can fix it. Right now I think the mechanical shifter engages something in the transfer case which presses the 4x4 indicator switch (on the side of the transfer case), which then sends a signal to the GEM to activate (or deactivate) the appropriate solenoid and that vacuum should land on the appropriate side of the diaphragm actuator, which engages something in the front differential to either lock or free the front axles. The GEM also sends a signal to the shift on the fly relay to engage the magnetic clutch inside the transfer case?

Also, I should have thought of this days ago but I just hooked up a obdII sensor and got the following codes: U1262, P0743, P0755, P1747, P0443, P1451

I am pretty stuck and would love some help figuring this out. Thanks again!