2010 F150 Lariat Supercrew 4x4, While in 4 High, the truck will occasionally make a significant POP. The sound is definitely coming from the front end and only happens in 4H and seems to only happen under load (no, I do not put the pedal to the floor right when I engage 4H). The front cv boots look to be in great shape with no signs of wear. I replaced the entire vacuum line last season with a brand new OEM line, a brand new solenoid and check valve, and had to replace one of the IWEs as it had failed. Today, I decided to pull the font wheels and see if anything else stood out instead of pulling the IWEs again to check their condition. I did notice that while the truck is running the front wheels will spin by hand in 2wd and 4wd... Now, I am not an expert when it comes to the 4 wheel drive system but this is not normal, correct? And where should I start my diagnosis to find the actual problem?

 

Makes a pop like maybe its not fully engaged?

Have you considered the transfer case?

First thing I would check is the 4wd transfer case solenoid which is attached to the transfer case.
It can get dirt inside the gears and prevent the system from fully engaging.

Pull it off and either replace it or take it apart and clean out the gears.

Its not an expensive part and its easy to get to.

Looks kind of like this...

 

also...

 

Do you sometimes hear more than one POP? It was years ago but I had similar issues with the a transfer case in a Jeep truck. The transfer case uses a chain to transfer power from the transmission to the drive shaft going to the front axle. If the truck is driven on pavement while in 4X4 the stress over time stretches the chain to the point where it starts to slip on the gears in the transfer case. It only happens when under a load.

My truck was full time 4X4 with the option of locking the transfer case for true 4X4. It used vacuum hoses to lock the transfer case. At some point before I bought the truck the 2 vacuum lines going to the transfer case got reversed. Meaning when I had the selector set for full time 4X4 the transfer case was locked. I was smart enough to know not to lock the transfer case on pavement, but not experienced enough to recognize I had a problem until I had to have the transfer case rebuilt. That is when they figured out the lines were reversed.

 

If it's definitely coming from the front under load, your IWE's (hubs) are probably worn and one is starting to skip under load. My son's '05 was doing that prior to rebuilding the front end.