Should I pay off a debt for a default judgement if I was not served properly?

Not being properly served, does not mean you don't owe the debt, it merely means you have a possible grounds to ask for a judgement to be overturned.
In fact at this point, if the judgement has not been challenged and they have full legal rights to collect.

You can dispute the service, and it will be investigated to see how it was done and who did it. There are various forms and ways of service, so they will just need to prove it was served according to state law.

But then they can of course merely refile and get a new judgement if you do over turn this one, assuming it is a valid debt

Why did you ask the same question using two different user names. Please only use one user name, use of multiple user names is against site rules.