Hi, I have a new "interesting" problem with my W123 300TD. When I'm at low speed and high torque, the car shudders. It feels almost like a flat tire or badly cracked pavement. Typical times for this to occur are accelerating hard from a standstill, or climbing a very steep hill. I'm not totally sure, but I think it tends to happen when the transmission is in 2nd or 3rd.

Other, possibly unrelated datapoints:
I get a little flaring between (I think) between 2nd and 3rd.
I changed all four tires so it's probably not to do with those.
Rear axles were replaced ~50K miles ago
Drive shaft bearing and mount were replaced ~50K miles ago
Flex disks were replaced ~50K miles ago
Transmission was replaced with a used one ~50K miles ago

Thanks in advance for your help.