Clunk in 4 wheel drive

[Jimamatic]

This year during hunting season I had my truck on several gravel roads. I like to drive them in 4 wheel drive to avoid back wheel hop and
exercise the system. On occasion after switching to 4 wheel I would get a big clunk that you can feel and hear. It's not the normal clunck you sometimes hear right after you switch. It happens long after and is usually on a moderate uphill grade at a slow speed. It's the first year
It's done this and the only difference is I bought 265 75R 16 tires for it. I've been around
4 wheeling my whole life and never had this happen. Any ideas?

2001 2500hd CC D/A 32000+ miles

[More Power]

The big clunk is likely either the front axle actuator finally engaging or the transfer case engaging 4WD.

Generally, the txfer case engages pretty quickly.

Cooler temps can delay front axle engagement slightly due to thicker lube, but it is an electric servo which is more reliable than the older thermo-actuator.

MP

[Jimamatic]

Thanks for the reply More Power! The temps never got below the mid 30s and I did put some synthetic front end lube in for grins. The question I have is would the 4 wheel light illuminate if the entire system is not engaged?
When that light goes on, I need to have the confidence that 4x4 will be there. Otherwise,
What's the point?
Jim....