I have a 2000 f150 4x4 4.6. When take off it sounds like a low geared bus. It sounds like I'm sucking air through 1st and 2nd....then it runs fine. What has me baffled is that It'll run normal for a week or two and I'll get excited it's fixed itself. Then I'll fire it up and it'll go back.. I've searched like crazy and can't figure it out

 

You must know by now what 4x4 Low runs like!
Is that what it is doing?
Is the 4x4 light or 4L light on?
You need to get a closer observation as to what it is doing before a solution can be found.
If you suspect it is 4L, a fault is causing the GEM to put the drive train in that function.
Normally 4L requires you to be stopped, foot on brake in Neutral to get in the low gearing.
Check for a windshield leak onto the GEM under the dash.
You have to find more info to get a direction on this.
A good fast check is to remove the plug on the transfer case motor and see if it still happens assuming it's not already in 4x4 mode.
Good luck.