Suspension clunk - goes away with weight in bed

[k2rm]

I have a 1970 C20, trailing arm rear, stock height with front disk conversion and font sway bar.

I have been fighting a clunk for quite some time. If I brake hard I hear the clunk. If I then accelerate, regardless of speed, it clunks. If I accelerate slowly or brake gradually, no clunk. It is more pronounced turning either right or left. I can also feel it through the floorboard when it clunks.

Yesterday I hauled a dresser, maybe 150lbs. No clunk! Somehow adding a little bit of weight makes it disappear. Any ideas of what it may be?

Thanks!