I doubt they will start letting people go until the deal closes. In the unlikely scenario that something prevents the merger from closing, they need to keep operating as normal.
They might not even start layoffs immediately upon close. Nobody knows yet how long it will take for them to decide on a new organizational structure and who will be going where. They could easily keep operations separate for several months after close to make sure the transition is smooth.
Of course, they could be ready to merge staff the day after closing and start the layoffs early... only time will tell but my money is on late Q2 or early Q3 when the axing really starts. Likely to be a much higher percentage of SWN folks hitting the road vs CHK. Only safe place is probably Southwest App since they need some experienced people for that business unit.
It certainly has been a good run, good luck out there!