FOLLOW UP:

​​​​​​​I was way off base. Got new tires, sound/clunk is gone...completely gone. Not sure why it would have been bad only in 2nd and 4th, but it is now smooth as silk in all gears all the time. I've put 1000 miles on it with the new tires and I've had zero noise, zero clunking.
In hind sight I'm sure the old rear tires each had a flat spot and depending on how the flat spots on the 2 tires randomly ligned up I'd get more or less clunking and noise.
in any event, it was totally just the tires.

 

gears had nothing to do with it, but it sure threw us for a loop.