My buddy was messing around in my truck while he was waiting for me (I hate it when people do this).

Anyways, he was dinking around with the 4WD and left it in 4-Hi, I drove about 5 mi at speeds of 35-40 mph before realizing what he had done. Is this bad for the truck?

It's not good/wise, but I don't think you have to worry too much in regards to damage. Just smack him in the face.

Just smack him in the face.

With a lugwrench. :naughty:

Hope it didn't wreck the transfer case shafts, bearings, sprockets or chain. You can check the fluid for metal flakes.

For only a few miles, at less than 40mph, you should be fine. Unless he was really hammering the gas from a dead stop, then you could get some really fast wear down. Just take your keys with you next time you leave him alone in the truck!

You can check the fluid for metal flakes.

Will do, but would I need to drain fluid completely to check?

Will do, but would I need to drain fluid completely to check?
A short trip should not hurt it.
But.
What year and what transfer case?
Some take the new GM transfer case fluid that needs changed ever so often anyway.

It's a 2005, 2Dr

If I remember correctly the transfer case fluid is supposed to be changed at 75,000 KM (I'm not entirely sure though). Currently, I am at just under 15,000 KM.