Could be coming from:

U-Joint
U-Joint bolts (no locking plates)
Front or rear yokes (worn splines)
Propeller shaft bushings
Clutch
Throw-out bearing
Pressure Plate
Transmission bearings or other Transmission part

In order of what to check First.

Illustrations
1 Torque Tube Passenger and 1/2 Ton Rear Axle
2 Universal Joint Passenger and Truck
3 Clutch Parts
4 Four Speed Transmission -Exploded View
5 1948-57 Four Speed Synchro-Mesh trans

1954 Chevrolet 3105 Deluxe Panel Truck
w/Factory HYDRA-MATIC