This is what a spool looks like,
it bolts to the ring gear and replaces the stock carrier. You don't get any clicking/popping noise with one as you do w/an auto-locker(ausi, detroit, etc.), you just get a lot of tire chirp/squeal when turning in parking lots, the same as you will with a welded diff. The only difference between a spool and welded diff, is a spool is stronger and it costs about 1/3 or half of what an auto-locker does.
With the amount of prep work you have done/plan to do, you should be giving yourself the best opportunity for the welds to hold. It will also give you an idea of what way you want to go if/when you decide to go with lower gears.
You are right, the welded diff will want to push you in a straight line, auto-lockers do the same thing. Turning in the rocks or on tight trails will also be more difficult, which is why I don't recommend beginners use them, it is hard enough trying to pick the right line and set themselves up for it w/o being pushed all over the trail. Granted with you and your buddies competition your situation is a little different than most.
