Thanks for this-
1. Mid perch. All work done by the guys at SDHQ in Arizona, so a reputable shop that knows what it is doing. No changes to the settings recently.
2. Ideally, I would use 4WD every day- I bought the truck as a toy for the desert out here in Dubai. In practice, it has been sick for most of the time that I have owned it (and I didn't go offroad at all for the first few months I had it due to an unrelated back operation…). Since October, I have used 4WD a couple of times per week. I have only done 5 or 6 proper day trips in the desert.
3. I have never engaged 4WD under load. I have engaged on the fly, but always with no power on.
4. The problem only occurs in 4WD and we have narrowed it to the front end. Do you mean gearbox problem or something in the differential? Ring & Pinion etc were all ok...
You need to make sure the vacuum servo is getting vacuum all the time when in 4wd. If possible get a separate vacuum pump and test the servo making sure it is not leaking. This is what I would lean towards