Front end rumble

99 Ford 350 4X4 115K Maine

Purchased truck used last fall. Soon after purchase noticed sometimes I would get a rumble under drivers front side when I would hit a bump or seam in road. Rumble would continue until almost reaching a complete stop, then rumble would “disengage” and be fine until hitting a bump just right again.

Inspected front hubs and found LF stock hub shot. Replaced both hubs with Warn hubs, sypmtoms fixed.

January, frost heaves, more bumps, symptoms re appear.

hub dissengaged/ 2wd trans:

Rumble occures sometimes louder than before

hub engaged/ 2wd trans:

No symptoms

hub disengaged/ 4w trans:

No symptoms

hub engaged/ 4wd trans:

No symptoms

My mechanic hasen’t a guess. I would like to save expence of exploratory surgey. Does anyone have any ideas what may be the problem?

Thanks for your time.