Newark
Assuming it's all on time you *should* be OK.....
We did Cancun - Newark -- Manchester a couple of years ago and it was
tight on a 3hr layover , but that was obviously going via Immigration, although
I must admit they were far more pleasant than Atlanta! And of course we had
to pick up and re-drop bags.
The inter terminal shuttle is VERY quick, however, we did find it's not the best
signposted service in the world! A quick review of the terminal map before arrival
and get an idea roughly where you want to head!
Having never done a domestic flight within the US I'm afraid I've no idea what
you need to allow for in terms of arrival / departure delays.
Regards
DaveA