You just need a 0-200psi pressure gauge in the L/R servo port with the vehicle running while in low gear. It's the furthest port back on the passenger side.
Would like to revise what I wrote.. if you have a zero reading you have a pump or inner gear failure, or the pump is not picking up any fluid (filter fell off, etc).
If you have a pressure reading (at least 100psi) and the car will not move backwards or forwards, you could've broken the turbine in the converter where the input shaft splines, or you have a shaft failure (unlikely at that power level) or other geartrain failure.
Was there a lot of debris in the pan, like a small bomb went off?