if you still need ideas:
did you have all 4 wheels checked? i'm thinking that perhaps the rear axle is in a perpetual right turn.
2nd idea
setting more caster on the right or less on the left could help
of course you would have to find the right shop to bend the I beam.
one last thought if for some reason all the work has resulted in a negative caster, like if the front is a lot lower than the rear, you could tilt the whole axle back at the top (more positive caster on both sides) this would give a bit more directional stability.
