If I haven’t received a W2 from one of my employers, should I input my information from from 2017 W2 as an estimate on what I made in 2018 (my pay hasn’t changed)

No, you should not. There are two options:

(1) File an extension and then file your return when you get the W-2

(2) File the return without the W-2 and then file an amended return when you receive the W-2. 

You are better off filing an extension until you can either get the W2 itself or exact information that is on the W2.  Worst case scenario contact the IRS to get a wage transcript.