I have old debt from the 1990s. Its not on my credit report anymore. I want to buy a house and I paid off an old judgement... today I got a letter about some of this old debt that was dropped off and they want payment and I was wondering if they could reattatch this back on to my credit report? The debt is a credit card from 1998.

Re-aging debts leave a paper-trail and hence make it easier for you to win a lawsuit.
If something has been illegally re-aged after it has gone past SOL, then you shouldn't be paying for it. You may inform them that they're pursuing a debt which is past SOL through a certified mail (return receipt requested). Within it, you may also mention that they have violated xx fcra statutes. Let them have 30 days to remove it. It will be looked upon as 'willful noncompliance' if they don't remove it (if it was done out of error they'd certainly remove it).

This is not correct. Old debts remain debts. After the SOL has run out the owner of the debt can no longer use the courts to force a judgement but the debt is still alive and valid. That letter you got is likely from a debt collection agency who bought the debt for pennies on the dollar. If you offer to settle for even a penny the SOL ends and a new one reopens so be very cautious what you say to them if anything. There has been no fair credit reporting violation from you have told us.