transfer case

If the vehicle were stuck in 4WD, I would expect the tires to hop and make little skids when you make a sharp turn on dry pavement. If you are not getting 4WD lockup then something else is wrong.

What does it do when you intentionally engage 4WD, does it act the same.

In any case, most transmission shops will do a free diagnostic drive. Might charge a bit if they have to connect the diagnostic computer.

I have a 93 Explorer with 5 speed manual. I am on my third clutch (2 replacements + original) and have 122k miles, they only last about 50k miles. you may just need a new clutch, it could be slipping. Do you need to go all the way down to engage the clutch, does it seem heavy.