1999 4×4 Blazer “klunk” in Drivetrain? NOT Joints.

1999 4-door Blazer. 4.3L Vortec v6 automatic with push button 4×4. Problem arose as a feint light consistant click at slow speeds. replaced Universal Joints. problem persisted and rapidly has gotten louder and progressed to a “double click” pattern. Now let me describe the “noise” a little better. at first it was consistant with vehicle speed produceing a clunk or noise every one secound at cruising speed of 55 mph. As you apply throttle or torque the noise increases in speed and loudness….until you allow the vehicle to cruise again. there is a slight vibration in the floor board every time a “clunk” is produced, and this also increases in audible volume, yet decreases in speed if you manually downshift and put reverse torque, or “engine brake”. problem persists in reverse.

Over the course of two or three days after replacing driveshaft joints the noise has progressed to more of a “Ka-clunk” sound…. hard to describe I suppose. does not increase in speed with RPM. only vehicle speed. problem persists in Reverse gear as well.

The only thing I have found on the internet is the possiblity of a poorly designed hydralic pump in the transfer case that catches a corner of the case and will start to wear and catch on the case housing itself. Too scared to stick it in 4×4 to see if the problem gets worse.

Let it be known that, at 55 mph with doors and windows closed, the sound seems to orient from the rear center of the vehicle, but with doors/windows open, it seems like it originates around the center underbody of the vehicle. It really really sounds like Universal Joints…but they were changed due to the noise, and it did not fix the problem.

Any ideas would be appreciative as it has now become a backup vehicle and can not be sold in its condition, as there is no telling safty issues or how long it will even drive with the rapid degeneration of symptoms.