I'm gradually debugging my Dart 451, A833, 3.91 8-3/4. I've discovered a strange vibration only in 3rd gear. Doesn't seem to matter what engine RPM or driveshaft RPM is. At a given road speed, shifting out of 3rd to either 2nd or 4th makes the vibration stop. Cruises without vibrating in 4th at 60 mph (~3000 rpm). I dialed-in the Lakewood with offset dowels.

It does not seem to change with accel or decel. Smooth in 1st, 2nd and 4th (straight-through, this is not the OD trans).

This worries me because I'm thinking it may be a countershaft bearing problem... but then why doesn't it do it worse in the first two gears?
Possibly shifter adjustment or a shift rod hitting the tunnel in 3rd? Any thoughts?

Put the car on jack stands or lift with the tranny in 3rd gear and remove the nut that holds the shift lever onto the shift shaft and sees if the shaft moves or not If it moves it is not engage completely, if it doesn't move let us know what you find to fix it please

I'm not sure I follow... the shift lever has a rectangular opening that indexes it onto the neck of the shift fork. There definitely shouldn't be any relative motion there unless the opening is wallowed out.

Or I could just disconnect the 3-4 shift rod without moving the lever, and see if it's possible to rotate the lever more towards 3rd gear engagement. Is that what you meant by "shaft moves or not"?

Adjust the shifter, the procedure is simple. While in neutral an 1/8 drill bit should pass through the
alignment holes in the shifter.

Pretty sure that's a 1/4" not an 1/8".

Yes, it's 1/4". I did that when I put it together, but will definitely check again. Certainly before pulling the trans

Might want to verify the shifter stops aren't mis-adjusted as well if they're present

Loosen the lock nuts, if your shifter has those stops before and back off the lock bolts before adjusting the shifter linkage with 1/4 inch alignment pin in place, the stops can be causing issues you wouldn't think of

No stops. It's a well-used stock A-body shifter.
Think it's this model (6353 reverse lever stamp): http://www.brewersperformance.com/proddetail.php?prod=SM-PRESS-R

Those little roller bearings between the main shaft and input. Think about it, more deflection at that “joint” in third gear than second or first. Other possibilities, bad teeth on gear or cluster or wrong ratio gear. Take out time and I'm afraid