I have been through the thread above and posted my problem in it to no avail. I realized it might not get as many views as it is in the technical articles section. I figured I might get more eyes/ideas here.
The truck is a 2006 fx4 crew cab with around 120,000 miles. I started having a problem where the 4x4 would disengage under load and then re-engage after a while when I let off the accelerator. For example: I put the truck in 4x4 high, light comes on stays on the entire time. I start to crawl slowly up a grass covered slope. It starts out fine, then as more load is applied to continue up the slope, I hear a bang/clunk and the rear tires start to slip as fronts are disengaged. I let off the accelerator and the tires re-engage. Repeat.
I read through the above link and started off by replacing the hubs and actuators as the gears on both looked a little worn anyway. The parts were replaced with the appropriate motorcraft parts. This fixed my ABS light but the 4x4 was still acting up.
Next, I replaced both of the check valves with the corresponding motorcraft part numbers. One of the old ones had turned into a two-way "valve" so I had my hopes up, but still the problem persisted.
So I bought a vacuum gauge and tested the vacuum system. Everything held and released vacuum per spec. I bought a new solenoid just to rule that out as well. I already had the updated part but figured what the hell.
I did read on here that the member Josiah was possibly having the same issue a few years ago, but he never really updated on the solution to situation that I could find.
I am thinking about purchasing a used transfer case off ebay for around $200 shipped. I just have a hard time believing the transfer case is broke. The truck hasn't seen heavy offroad duty in about 3 years and the problem just popped up last winter. Before that, it was just Silver Lake sand dune action. Nothing too hardcore, no submerged water crossings or jumping.
Any thoughts or ideas would be greatly appreciated.