1999 F350 7.3 dually
It all started when my heater would go to defrost when I selected 4wd, then I noticed hub would not lock in auto (manual is good).

Neither hub will hold a vacuum

Removed line running to hub and plugged vacuum source, vacuum pump runs continuously (2wd/4wd).

It appears I have more than one problem but I want to fix the hubs first as I drive through a lot of mud, water, ice and snow.

I have read and downloaded Diesel-O-Ring and I am ready to start, truck is up on the hoist.

Here is my question. I thought (maybe I had a senior moment) but isn't there a seal that can only be changed by removing the differential 3rd member and using a special tool? I swear I saw this in a thread here but I can not find it, Guzzies write up does not cover it and my Haines manual dos not mention it.

 

Fix the leak in the vacuum supply circuit first. Without a sealed supply, the other symptoms can be masked or appear ambiguous. The PVH solenoid is a common failure (the housing splits or cracks) as well as brittle vacuum hoses.

You can always manually engage the hubs if they are needed.

 

I will be trouble shooting the other stuff while I am waiting for parts. My main problem is getting my ducks lined up for fixing the hubs, If I have to remove the front axles to replace seals, per Guzzies write up, I might as well replace the one (if it exist) that requires removing the differential 3rd member.
The truck will not come off the hoist until all is fixed, the kind of driving I do the auto hubs are wonderful, when they work and I do not want water and mud destroying my front end.
Thanks for the response

 

OK I "gotter done"
My original question was about an inner seal in the axle tube. There is what looks like a Teflon seal midway in both axle tube, to replace these you need to remove the differential third member and you need a special tool. This seal appears to be there preventing diff oil from leaking out at the hubs, as my tubes were dry I did not touch them.
Tips:
1. If your vacuum pimp is running constantly (as mine was) that is a very good indication your vacuum pulse solenoid is bad. Diesel O-Ring has them
2. If you Select 4WD and your heater position goes to defrost, one or more hubs are leaking.
3. Replace ALL seals and O-rings. Diesel O-Ring has them
4. There are 2 bearings per side, replace both of them. I found that even though I was not having any front end problems, the needle bearing (on one side),that many say can't be replaced was bad. With a straight punch and a little care not to mar the race I knocked it out in less than a minute. The bearing in the hub assembly I pushed out with my fingers. Replace all 4 bearing, my cost was $32 total and the part number is on the bearing.
4a. Before replacing the needle bearing make sure the axle stub shaft race is in good shape
5 .I recommend getting a really big three jaw puller, especially if you live in the rust belt, I don't but it made the job really easy.
6. There are several write up (see Race Guy) on how do do this job and make the special tool required, they are excellent, there are also some youtube video that are good.
7. This is a good time to replace the rubber hose that runs from the frame to the hub.
8. Last, I installed the hub actuator assembly on a very cold morning and it would not disengage, too much grease. I cleaned out most of the grease and sprayed a little WD40 and it worked perfectly.