Can I take a domain pc hard drive and put it in the exact same hardware and leave it as it is on the domain without running any sys prep?
This won't be an issue. You only need to sysprep if the machine is going to be cloned and used side-by-side since then both machines would have the same domain SID, which will cause logon problems. If it's a straight replace, there's nothing to worry about.
I wont be a problem. Swap it..
Only thing you will probably run into is windows activation issue (sometimes).