Transfer case problem?
 

I have a 1997 Chevy s10 zr2 with the 4.3L. 280K miles, 100K on the engine. I've noticed sometimes when I go to engage the 4wd that I get a harsh clunk after moving a few feet. I don't get the clunk when driving or anything, just in that initial moment. The 4wd works like a champ otherwise. Is this anything to be worried about or is it just because it has so many miles?

Is the noise when engaging into 4wd low....
And a 3 button transfer case.?
There is a noise in that inital engagement....
But I keep mine at a minimal by shifting at a
Total stop, then a very slight forward movement.
Also has the transfer case fluid been checked?

Actually I meant when going into 4 hi. Yes it is 3 button. I checked the fluid and it was overfull due to a leaking tc input shaft seal which I will be taking care of here shortly.