There's quite a bit of difference between the two shoes...the GT-2110 has a lower, softer heel than the Nimbus, and as Tom said, the GT-2110 also features some support/stability through the midfoot (arch area). My guess is that the lower, softer heel of the GT-2110 is causing some extra stretch to you lower leg/calf muscles. This in turn is pulling on the achilles, causing the discomfort. You need to really stretch your lower legs, the muscles of the calf, the achilles, and even the arch...everything is connected.
Shawn
TORRE Consulting Services, LLC
http://www.TORREcs.com