Transfer case 4LO not engaging?

Hello!

I listed this truck as having 350,000 miles, but this there is a caveat. The Carfax has some unclear information about it, and we really don't know the mileage. The frame and underside of the truck are in great condition. No rust whatsoever.

But regarding the issue I'm having.

I recently purchased this truck. The previous owner had just replaced the 4x4 actuator. And the 4x4 was working when I purchased it.

Now I am having some strange symptoms. The truck shifts fine into 4Hi, but when I try to go into low range, I run into problems. I am following proper procedure (go into neutral, etc, ) according to the manual, but when I shift from 4Hi into 4Lo, one of two things happens.

1) Occasionally, the vehicle goes into freewheel. Basically neutral. From my research, there is a "freewheel" section in the differential between high range and low range. So apparently the truck is getting stuck halfway between high range and low range.

or 2) more frequently, I hear a humming sound coming from the 4x4 actuator. When I put the truck in gear, the humming sounds turns into a buzzing sound that sounds like the plastic gears in the actuator are grinding in an attempt to shift into low range.

It appears to me that the transfer case is having difficulty shifting into low range. This is probably what caused the previous actuator to go bad. And it looks like it's doing the same thing to the new actuator.

My guess is that the 4x4 on this truck has rarely been used. And I don't know if the transfer case fluid has ever been changed. So, the components in the transfer case may be seizing up from non-use and low maintenance levels.

The only thing I know to do is to check/change the transfer case fluid and see if things start to move more freely. I may have to replace the 4x4 actuator again as well. But I would value any input from those with more experience.