1999 f150 lariat 4x4

So i bought this truck a few months ago and have been having problems with the 4x4 since i got it. When i first had it, the 4x4 would only shift into or out of right at startup. If i waited too long then i had to shut the truck off and start it up again to try again. I think that was due to cracked vacuum lines, but i wasn't ever real sure.
Then one day it just started to work, i mean perfect. It shifted to what i switched it to when i told it to. It was working for about 5 hours when i had it parked and running in 4x4. I hopped out of the truck to talk to a buddy, and hopped back in the truck and noticed it had shifted out of 4x4 (no dash light), even though 4h was still selected.
It hasn't worked since then.
I ended up swapping out all that plastic line crap with rubber vacuum lines. All lines from the front drive-shaft to the actuators to the vacuum resv. Now i have a constant vacuum going to both actuators like the vacuum diagram shows i should.
The problem is that no matter what the selector is on (2wd or 4h), i only get vacuum out of the 2wd solenoid, ever. I no longer hear the familiar click of relays behind the dash when I'm trying to shift. I know the relay is in the top of the dash, and the gem is behind the fuse panel, but I'm not sure which would be the more like culprit. I'm not getting a signal to my solenoids telling them to ground out, so 2wd is always engaged. Not sure what or where to test next.
Any help is greatly appreciated.

 

Have you checked the GEM for corrosion?

 

Going to remove the kick panel within the hour to look. Checked out the windshield last night and couldn't find any evidence of leaks but im not a windshield pro either. And for all i know it could of been replaced at some point. After i swapped out the vacuum lines, i ran out of time. I'll let you know what i see when i get under there. Am I looking for a corroded connector attached to it?

 

Yes, in addition to corrosion evidence on the whole box - it may be corroded inside.

 

Well i got the kick panel off, but couldn't figure out how to get the fuse box dropped out of the way. From what i could see though, the GEM doesn't look to be in bad shape. I couldn't see any forms of corrosion, or evidence that anything had been wet at any point.

 

Please don't take insult to this but as help to you.
Your first post does not make sense for the operation of the system for several points made..
You clearly do not know how the system operates.
Short primer:
The dash switch request the GEM to operate the transfer case motor to it's first position.
This engages the transfer case to power the front drive shaft.
At the same time it puts a forward circuit to the firewall solenoid to transfer engine vacuum to the front axle actuator, locking the front axles together.
The firewall solenoid returns a signal to the Gem for it to light the dash 4H lamp.
************************************************** ***********
As of the moment it sounds like you have no power to the case motor or the GEM is faulty. The relay you hear under the dash powers the case motor both ways.
At this point you can't get past operating the Gem to perform these functions.
Only way the system operates 'it'self' falsely is a faulty GEM. It does not depend on any signals from the firewall solenoids.
Reason is you cannot afford false operating while at speed, especially in 4L.
Good luck.

 

Got ahold of a high dollar scanner. Pulled codes from the gem, 1867-contact plate malfunction. Replaced the shift motor, which looked rusty as i could get out, and low and behold, 4wh and 4wl work again. Hope this helps some one else, off to tear up some pits.