Troubleshooting Transmission '47 pickup

[GB SISSON]

Last week while heading down our steep gravel road in first gear, the floor shift knob started vibrating badly and there was a loud clicking or ratcheting coming from the transmission area. There were no previous indications of this condition. This truck is the one I converted to a woodie wagon a few years back. I got us turned around and the noise/vibration was not evident going back up the hill. Later that day I pulled the driveshaft and checked the splines and u joints. Splines were perfect and showed almost 3" of engagement. The u joints were also in good shape. I finally got a chance to drive the vehicle quite aggresively today and the sound is very pronounced in decelleration and non existant in acceleration. Another important factor is this problem only exists in first gear. Second, third and reverse operate as before. While I have completely overhauled six or seven flathead V8s and all manner of other repairs on the V8 trucks, I have no experience with transmissions. Is it time to pull this 3 speed and open it up? (I have all winter). Could this be something more external? And lastly I have been considering a T-5 swap as my tall tires, hilly island environment, stock 221 engine and heavy vehicle have me longing for a lower first gear. With open driveline, maybe the T-5 swap is pretty straighforward? Thanks in advance, GB