92 front diff engagement?

Hi Folks,

First time to this board. I recently picked up a '92 K1500 with 165K miles all stock and have noticed a situation with what I beleive is the front diff. Most notably on the highway.

When cruising at highway speeds I often feel a hard "clunk" and can almost detect something new spooling up with an immediate/initial drag on the vehicle. Almost like the diff engaged by itself. I can never feel it disengage. The FWD works like it should when selected. I have tied a cord around the front drive shaft to a frame piece to see if the DS is engaging. Nope, I drove 300 miles feeling the clunk often but the DS never rotated. This situation may also be happening at slower speeds but may not by as noticable due to the speed. This clunk is not effected by bumps, accel/decel, or turning. The fluids all changed in the diffs and trans. A/C compressor unplugged and DS u-joints good. I've been an engineer in the bearing biz 20 years and am sure not the wheel brgs. It does not feel like a trans slip or shift and it happens in both 3rd and OD lock.

Question: Is there anything in the front diff that could engage by itself and not engage the front drive shaft? ALL input is welcome.

Thanks,

Gary
Milw, WI

Have you checked the rearend? Might be something in there, sounds similar to my woes when I blew my rearend up all over I90 in chicago.