Mechanics haven't helped... can you

I have a 1999 Expedition 5.4 a4wd. I have worked on all my cars for 30 years as well as in shops but never consider myself a mechanic but usually diagnose things pretty good. This is something that I haven't had before. While driving at 50mph plus if I hit a little bit of a hill and I hit the throttle the truck feels like the 4wd engages and just falls flat, I can hear the engine struggle. If I let off the gas slightly it clunks, pulls to the side a little and lets loose and the truck feels normal. as soon as I step on the pedal again (the slightest touch) it does it again. It doesn't feel like torque converter shudder but it feels like trans/differential. I hear what I think is a vacuum leak or almost the slightest grind when I am just at idle. I know there is a vacuum leak somewhere because under load the AC switches to Defrost...My mechanic thinks its actually starving from fuel or spark and its the engine that is sputtering, which might have some truth to it but the bog down in power and clunk/pull tell me it's drivetrain.

If it is a fuel issue.....easiest way to test is to read the rail pressure w/ a gauge.

See what it is at idle, and if it can be monitored while driving, that would be idea.
Otherwise, put it in gear and give it some gas and brake, see what fuel pressure does.

This is only a guess, I believe the 4WD system is engaged when the vacuum at the hubs is removed ie. atmospheric pressure is present. I believe the vacuum to enable the 4WD system is taken from the inlet manifold system, so when you put your foot to the floor, the throttle body open fully and the vacuum would disappear, allowing the 4WD system to engage. I believe there is a 1 way valve between the 4WD system and the inlet manifold, to prevent the vacuum from completely disappearing, maybe that valve has failed and is allowing the vacuum pressure at the 4WD system to reach atmospheric pressure and engaging the 4WD.. it just a guess, but i would start by locating the one way valve and test it by trying to blow through it (with your mouth not an air line) from both sides. It should only allow you to blow through it from one side.

Drive it in aFwd and see if it changes.

Does sound like it’s a vacuum issue that’s partially engaging your 4wd. Using carb cleaner, spray around the engine bay and 4wd actuators to see if idle changes. If so, there’s your leak.

I agree with garry, check the oneway valve ,there cheap just replace it ,also check the soleniods ,they are on the passengers side under the hood at the bulkhead.

The small hard plastic vacuum lines break, I repaired a similar problem with a hard line on my 04 near the battery. I think the 97-02 expedition have the same front axle lock setup as the 07–03 F-150. The F 150s have a vacuum operated shift fork on the front differential. The shift fork slides a coupler on the right side output shaft to make the front differential spin. Dodge and Gm have similar front axle disconnects, they all have their failure points.