OH MY GOD! YOU BROKE IT!
nah im just kidding
do you have a digital camera or camera phone so i can see the steering setup? its pretty straight forward, theres not a whole lot that can go wrong without being relatively obvious. if i had to guess, you either used the wrong hole on your servo horn, or you set it up with the servo not centered, and used your radio or turnbuckles to compensate. when my wheels are neutral, i like my servo horn straight up and down. that way i know exactly where center is. the stock steering horn turn buckle (the push link) may not be long enough to put your servo horn at 90 degrees, so just use your best judgement to make sure everything is where it should be. i know you said youve had experience with touring cars before, so stop me if im patronizing you
im reasonably sure that its somthing simple.
as for the wobble, that could be one of 3 things. one, make sure the bearings on both sides of the rim are seated properly and in working order. two, make sure the bearings are actually there. ive gotten GTs second hand where there was only an outer bearing, and missing the inner ones. you need both of em! and three, I bad glue job will cause that too, believe me, im an expert at doing bad tire gluing jobs
a lot of the time a few laps around the track will help normalize the tire and stabilize the wobble. worst case, search the forum for how to remove tires from their rims, and reglue them.
on to the diff... That sounds like the diff is just loose. remove the rear dog bones or CVDs and inside one of them is a little white cap (INSIDE the outdrive). remove that cap and that will give you access to the diff bolt, which adjusts the tension on the diff. tighten it down until the spring in there fully compresses (until you can no longer turn it using moderate, but gentle pressure) then back it off 1/4 of a turn. that should snug things up
good luck! any questions, feel free