I just want to be sure - the PXE server and DHCP server are still the same, just in a different location. Clients are getting IP addresses (so DHCP is working) but PXE is broken now, right?
I am assuming this is a DS 6.x server, correct? You may simply want to pull PXE using a custom installation and reinstall it. DS 6 is pretty easy to modify this way. It's possible there are some INI's or something that need tweaking. I know in the past I used to know how to do this, but that was ... 6 years ago? No longer!
Alternatively, there are those who are in advanced support that probably know how, but finding then may be tricky. Others may know what to look for, but I'm rusty on who has those skills there now.
So, were it me, I'd look into pulling PXE off the box and putting it back on. It's not like the 7.5 world where that breaks everything. It should be pretty easy, but if not, well, let us know. That is, it should be available via menu during the DSSetup and I believe a custom install selection.
At least look. If you have a lab, you could test it there first. My DS 6 lab is down... :(