I’m using an original 1955 differential in my Bel Air. Code AC1001, 4.11 gears, originally used with 3 speed manual w/overdrive. I just converted to a fully rebuilt 700R4. When shifting from park into drive or reverse, a pronounced clunk is heard and jerk is felt as the driveline engages. It does not seem to impact drive-ability. No weird noises or vibrations up to 70mph. On further observation from underneath, the driveshaft can be seen to rotate approximately 1/4 turn at the differential carrier whenever this shift happens. All u-joints are new. The carrier had ring and pinion adjustments performed by a reputable shop 2 years ago (maybe 500 miles), including all new carrier bearings. When using the original 3 speed, there was always some backlash when going from from coast to accelerate and visa versa and I’m probably seeing the same issue with different symptoms (stick vs. auto). My thought is an aged ring and pinion, but before throwing parts at it I’d like to narrow this down. Any suggestions appreciated.