Did you have 3 strength points due to the previous cuirass? If unequipping it was the reason you no longer meet the stat requirements for Ulthring's Cuirass, that would be why it isn't showing up on your character. Try using a strength potion (any level), and see if it appears. Also, in that case re-equipping your previous cuirass should get you back up to 37 strength.
Ulthring's Cuirass exists in the original release of D2:ED, as well as DKS / D2:DC. With the DKS release the stat requirements were removed from equipment, so there was just a possible level requirement.
The stats for the set were changed a bit between D2:ED and DKS, so that is likely what the site was referring to.