You've got a WALK??
If not, get the Superpro version (much cheaper and arguably better). The front wishbone mount isn't as essential, but it'd be daft not to do it too.
A full set of mounts is overkill, I can't help but feel you've been led down the garden path there if you were sold them on the basis of them being a wheel hop cure. I'd class them as a 'nice to have' and not a 'need'. I certainly never felt the need to have them on mine, and that got some serious abuse on track.
To check the diff, axle stand the car and put it in a forward gear on tickover. Both wheels will rotate in the same direction if it is working ok. For the second part, it being faulty under load, you'd maybe need to seek some expert advice there, but it'd been a really really slim chance of it being the cause, so don't focus on that too much.
Oh, and mine never pulled to the left.
You've had the rear geo checked, right?? They're way out from the factory, straighten it up and it'll track true.