I had similar issues with ours Envoy. At first, the 4wd would sometimes engage and sometimes it wouldn't. There were times it would seem to pop in and out. 4 lo worked when 4 hi didn't but eventually lost 4 lo too. When I had it completely up on jack stands the 4wd would engage but only after really getting on the throttle. I tested it out in the wet grass and there was no 4wd at all under load. I first tried messing with the disconnect. I installed new bearings and put new grease in. The gears are in great shape. I also installed a new actuator. Then figured that it was the clutches in the transfer case but that wasn't the reason either. I finally decided to try and manually engage 4wd by removing the actuator and pushing on the fork in the disconnect with a long extension. It took a lot of force and I could tell that the splined collar didn't want to mesh properly with the other gear. I did this by jacking up the vehicle, front and rear, removing the actuator but leaving the harness hooked up, key turned on but not started, and selecting 4hi. I ended up making a little gadget that would manually engage the disconnect so I could at least narrow down the issue. (I'll post a pic). After doing so the 4wd now works fine. I really don't see how the disconnect could be the issue because I basically rebuilt it. What I think is going on is that the bearings in the front diff, especially on the intermediate shaft, are bad. I figure if there's a slight wobble in the intermediate shaft that would keep the splined collar in the disconnect from wanting to slide over the other inner gear.
If I were you, after doing what others have recommended, I'd try and jack the vehicle up and manually engage the disconnect. Do so by leaving the key on and engine off. Select 4hi, remove the actuator but leave it hooked up to the harness. Press on the fork, or where the actuator plunger pushes, with a long extension while turning the tire by hand. The reason behind this is too see how hard it is to get the splined collar to slide over the internal gear. If it doesn't slide right over I would then rig up a way to where it's always manually engaged. You then have to be sure to wire the actuator up and in a safe place before test driving. Like I already mentioned, mine just doesn't want to mesh up. That could explain why it was jumping in and out of 4wd.
Here's the gadget I made to ensure that 4wd was engage. I used a piece of steel and a small piece of 3/4" black pipe. You could probably even use a piece of 1/4 plywood or a piece of plexi glass. Then used a piece of 3/4" pvc. It would be easier to work with. I ended up welding the pipe to the plate steel because I plan on just leaving mine for the time being. If you're just using it for troubleshooting just make it out of something basic and materials easy to work with.
One other thing, I remember someone saying that there are times it seems like the front actuator is working. You can hear it whining, and when removed, you see the plunger extending but the plunge isn't extending as long as it should. If you end up needing one you can get one on Amazon for $50 rather than spending $100 at the local parts stores.