My 2004 chevy blazer clunks only in 4whl drive, right front. Anyone know what it might be?

There is so much turning up there that it can be several things. The normal Balljoint problem can have the part slip in and out of its socket if it gets loose. Check your tierod ends for slack.
Sometimes Calipers can rock if they are failing to float properly with loose hardware. The front torque is changing from being pushed by the rear wheels to pulling with the front wheels.
This can affect how the Balljoints react as the sockets are getting torque differently than in 2WD. You are also using the CV joints to pull rather than coasting in 2WD. Check your end play for the front hubs; they used to be about .005 and you need Year model specific Specs to be sure.

Check out some 4-Wheeling Blog sites as they may know more about the common problems. There are so many things to check that a Blog may be your best source.