[justcuz]

Wow! I would start at the front and work your way back tightening all the driveline mounting bolts and look and see if there is an obvious rub somewhere. It seems weird that it changes with a bump. I'm going out on a limb here but do you think the previous harmonic may have loosened the 5th gear bolt in the transmission? Maybe a u-joint needs more grease. Maybe the adapter housing from the trans to t-case came loose or adapter to t-case. The last thing would be the throwout bearing maybe? It is hard to think of stuff. (I am trying to visualize in my mind your running gear). Is it directly related to vehicle speed or powertrain speed. If it happens again and you can safely do it, I would depress the clutch and throw it in neutral and see if it is isolated to the driveline or the engine. I had a input bearing crap out on a transmission once because the bell housing was not perfectly centered. The bell housing had been on a truck for years with no wear to the trans, but when I put the same engine and bellhousing in a passenger car with a Saginaw 4 speed the input bearing crapped out. I did remove the bell housing to clean it and reinstalled it, not checking the runout. My point is I ended up with a little rumble I could not figure out untill the transmission started shifting hard. Then I figured the bearings were getting loose. I have no idea what is happening but I hope you find it. This must be driving you nuts by now.