I Agree with JB this isn't torque steer, and I would hazard a guess that fitting a LSD to a car that did this would only make it worse (Just look at the new Focus RS, wanders like a fatherless child on anything other than a billiard table) I would say this nervousness under power was down to you suspension setup, low profiles and lowered uprated suspension is going to make a car more jittery. You say you changed the wishbones recently, but what about the topmounts?? wear in them could mess up you castor, toe-in/out totally, and be changing constantly as you accelerate.
As for the Quaife, you will be looking at a minimum of £500 + vat if you can find one for a 155. And they are a form of Torsen (Torque-sensing) diff, a purely mechanical unit that distributes torque to the wheel with the most grip. They work well on all surfaces, hense the reason they are in the rear diff of Integrales (6 world consecutive world championships) and Q4's They work fairly instantaeously, a feugusson on the other hand works with a fluid and disks, as the disks slip in the fluid, the fluid thickens and locks the two shafts distributing torque.
Oh and just for the record, and this is purely my point of view so please don't feel the need to comment on it, any twisting in the shafts is going to be negligible and totally unrelated to this problem.
Car companies spend millions on R+D to cure torque steer and the symptoms you describe, and then people wander why they get probs when they mess around with the setup. If you want to cure this, put the car back to standard, I'm sure the prob will go away, but in the words of Hunter S Thompson "buy the ticket, take the ride"