3 door cars have less castor than all other sierras, so will suffer more.
Most of it will be down to the tyres, try an extra 4psi in them.
A trick is to get some washers(3mm thick) with a hole drilled 28mm in them, cut accurately in half, then place between the tca and roll bar so they get butted against the stop to hold them in, this pushes the wheel further forward creating more castor.
On a personal note I have always fitted 3 door roll bars on my sierras, my last had a AXLE TIGHTENING KIT from GGR with Std ford escy beam bushes, and was the best setup up yet.
AVO will not cause this problem.
tabetha