No need to buy a new ECU, just remount old axle's sensors

Here are some approaches to resolving the issue:

The way that didn't actually work, though I hoped it would.  Another poster suggested that the unit might be reprogrammed, as per the manual, Unfortunately, the reprogramming is only for the presence/absence of stability control, not for the presence/absence of an axle.  The manual states this clearly, but the terminology only makes sense if you already understand all the acronyms.

The way that would have worked but would have been expensive.
Several posters have reported installing a new ECU that is set up for a 2 axle truck.  As far as I know, this really seems to work.  Unfortunately, the part lists for $1700 and is about $1000 even from "discounted" suppliers.  Some were available on eBay for $399, but as far as I know, these were not the "correct" part number, although they did apparently make the ABS work again.

The way that seems to work and doesn't require buying anything!
Fortunately, going back years in the forum, I stumbled across another poster's posting with the solution that I've now used.  My truck's ABS seems to me to be working perfectly now, as is the cruise control and engine brake.  The ABS light functions properly, and I have no "messages" in the computer.  The solution I used was as follows:

1) After removing the middle axle, took its two ABS sensors and mounted them on the rear axle's wheels.  Apparently there was space for two such sensors on each wheel, 180 degrees opposed to each other.

2) Hooked the ECU back up to the remounted ABS sensors.

3) Hooked the ECU back up to the (now useless) ABS valves that used to drive the middle axle.

4) Didn't need to hook up any air to the middle axle ABS valves.

5) Smiled when it all seemed to work properly!

In other words, the ECU is still hooked up to all the parts it used to be hooked up to. But the sensors that used to be on the middle axle are now upside-down on the remaining drive axle.  And the valves that used to drive the now-removed axle's ABS are left on the frame and still hooked up to the ECU (but they don't need to be hooked up to air).  So the ECU sees all the sensors and valves that it wants.  And its efforts to try and unlock the "phantom" axle don't actually do anything, because the valves it uses for that aren't connected to any air.

It is possible that the ABS doesn't perform in the same way that it used to, but it *seems* ok to me.

Disclaimer: I'm IN NO WAY a mechanic, and there may well be unforeseen safety and/or performance related consequences to this approach.  I'm merely reporting what worked for me.  I hope it works for you too, but IT IS UP TO YOU to decide whether this technique results in a safe or unsafe vehicle.  I have no way of doing that for you, and can only judge for myself.

Dan

PS: Some may be tempted to just "frame-mount" the sensors, and re-attach to the ECU.  I've not tried this, but I don't think it will work.  This is because the ECU is supposed to detect when the wheels are skidding, and it will disable certain things such as (I believe) cruise and engine brake under those circumstances.

PPS: Some may also try to just "splice" the remaining axle's sensors into the ECU in two places rather than just one. I tried this.  It doesn't work.

PPPS: If you have Bendix rather than Wabco, all bets are off.

No reason while this shouldn't work that I can see.  As long as the sensors see the wheel turning, everything is normal - if they see the wheel skidding, both sets of sensors will activate their valve sets and do exactly as they should - just one set of valves isn't doing any work.  I would think this same approach would work for ANY system - correct inputs, correct valves, just no actual air to brakes.  The only thing that could screw this up is if there would be a pressure sensor actually looking at the air pressure in or on the valves to the axle somewhere.  And if there is ,
THAT could be crossed over to the other axle as well.

There probably is a way to make one set of sensors feed 2 inputs, but it would require a electronic gadget and some study of all the parameters to make it work.  Your way is better!