I'm struggling to see how the solenoid failing could get the diaphragm to stick shut, unless it's actually the valve significantly leaking air past the seat.

Hey Gordon. I am by no means an expert on this system and frankly I feel like pouring petrol on the ** thing. I suspect my exhaust valve is causing most of my issues but I don't have a spare to swap. This is what I think is happening.

1. ECU opens both front solenoids and Exhaust solenoid (Only plunger is stuck or solenoid does not operate?)
2. Air is forced past rear plungers into rear bags.
3. ECU thinks front has lowered and opens rear solenoids and Exhaust solenoid (Only plunger is stuck or solenoid does not operate?)
4. Truck freezes with ass in the air and dash lights flashing.
5. Ecu thinks its stuck and raises to extended.
   The symptoms described in my last post are probably a mix of both Diaphragm and Exhaust solenoids failing. I have a used block on the way to rebuild and drop in.

The exhaust valve solenoid can be swapped with an inlet solenoid. If you can stand faffing about with it, you can swap them over and test. Will only cost you time.

I'm just going to wait for the replacement to get here. This is a picture of the exhaust solenoid that is in there. I think when it cracked it shifted sideways and bent the valve below causing the plunger to stick when operated.
I would rather a truck that won't go down than one that won't rise! lol nothing sexual intended!

Check you've got the valve base screws in the right way. That looks like something has stopped the valve body from going all the way down when the screw was tightened.

I was not under the impression one could locate the screws incorrectly. One set of holes goes through the base and one does not? One set of screws are countersunk and one set is recessed. I will look at it after replacing the valve block tomorrow.

OK I found the issue after a lot of work on the truck on the weekend.
First red flag of the procedure is it took 10 depressurizing cycles to get the truck down near bump stops. I had to remove the pipes to get it down all the way.
I replaced the valve block with a unit I bought off eBay and rebuilt. At first the compressor seemed to run forever so I knew I had issues. Off I went to get the soap and water spray. I found no leaks but discovered the previous owner of the valve block had mixed up the solenoids. Crap, depressurize!
Ding ding round two after checking all the wiring on the valve block. Truck rises but compressor does not shut off. Red flag number two. Checked for leaks and nothing. Compressor is still going. Getting hot so I pulled the relay.
After some head scratching I got the idea to check the dryer for a blockage as it seemed to have an issue getting air out and into the system. Dryer was done. Rock hard. New drier being ordered but I did find some desiccant to use in the mean time so I can keep truckin.
I have to rebuild the compressor again as it was being fried trying to push air past the block of desiccant in the dryer.