Error P1618 after transmission swap
 

So my transmission went out in my 1999 Volvo C70 2.4L Auto. I had the transmission replaced by a local shop. They put in a used one. Now I have the vault code P1618. The shop said it had to do with the "input speed sensor signal" but after some searching it seems like this could be a TCM to ECM connection error? What I'm wondering is if there is anything special this mechanic missed when making the swap? Any help is much appreciated.
Thanks.

GOOD INFO. Do you have the definition of the code P1618 on a 2001 Volvo - JustAnswer

Thanks for the reply. That information does help. To give some more background to the problem the car also has surging during acceleration and sometimes even holding speed. Its always MUCH more pronounced when I am accelerating from a stop.