Tranny swaped 98 Jaguar XJ8 New tran Giving stall Speed code

[dip2000]

Hi i just picked up my 1998 XJ8 after putting a new tranmission in it and im getting a stall speed error code the car takes awhile to pick up speed in drive the mechanic was thinking that the rear axile was bad but I scanned the car and got stall speed error and the dash says transmission fault. The fluid is full, is this caused by the speed sensor? Does anyone know what is causes this? It reverses fine but takes time to pick up speed in drive.