Can't engage 4wd

Could be right.
4by's with an electronic diff lock utilise a motor to drive a cog set in the transfer case.
If the motor is never run then its grease (plus the gear set that goes with it) can go hard and goop up which would initially show up as a very slow engagement and disengage. This can also burn out that motor.
Very common problem in Toyota 4bys and the motor assembly is incredibly expensive.
6 year old truck that is seldom in 4wd could easily have dried up.

Whatever, get it seen to quick. Last thing you want is to be stuck in locked 4wd mode for a long drive home as it can do serious damage to your front end drive train.
I got stuck in 4wd in the Brindabellas. Middle of winter on a Sunday when everything was closed. Had to drive home to Sydney at 40Km p/h. Took 7 hours to drive from Canberra to Sydney.

Not sure about the Colorado but my Triton suffered the same issue untill I read the manual and it said to run it in 4WD at least once per month. Never happened after that.

The same thing happened to my 07 Colorado (in 07). I took it to a dealer who gave me a printout and manipulated the three buttons in some sort of sequence (I can't remember - sold the vehicle in 07 - it was a beauty and never let me down after that. It went everywhere). I don't have the printout but I am sure if you approach a dealer they should be able to point this out. Hope this is of help.

Take your car marroning - it's why you bought it. cheers jrzoo

I had a 1993 Rocky with push button electric operated engagement, it got mud into the vacuum operated gizmo on the side of the gearbox, would not engage, I took it to the service center in Moorooka , they replaced the part at cost of around $290 (part and labor) , the part was left on the passenger side floor as requested, I took the end off, cleaned the mud out and it would have worked ok.. ...so maybe you have same problem.

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