A few common causes are:
It may fit, but it will not shift as they have different governor types.
if it is automatic then this would be caused by shift down is not working in transmission
Inside tranny
If they are Automatics it will not shift properly. It would bolt up if they are the same engine.
Yes it will... you need to switch the flywheels though to mount it to the automatic transmission...
the transmission went out
TCM, is the Transmission Control Module. It controls automatic transmission shift points, etc.
dirty tranny fluid
On the valve body, inside the transmission.
It may fit, but the control systems are different and it will not shift.
The shift control systems are different. It would bolt up, but it would never shift.
More than likely your shift cables need to be lubricated.