You should definitely find someone to solder for you, or pick up a decent iron and do it yourself. The companies that sell Lipos with Trx connectors installed charge way too much. With the prices at Hobbycity you could prob save enough to buy a decent iron and do it your self.

These packs have them installed, and they perform well. But they are way overpriced if you ask me.
http://www3.towerhobbies.com/cgi-bin...?&I=LXWSS0&P=0

These will need a connector, but they perform just as well and save you money.
http://www.hobbycity.com/hobbycity/s...idProduct=9437

As far as one wheel spinning faster than the other
The differential in the Slash is a gear diff. Whatever wheel is unloaded will get the majority of power, causing what you experience. The fix for this is to remove the diff and pack it with a heavy oil or grease. This will give more equal power to each rear wheel. The problem is that the diff is not sealed, and the lighter the oil or grease used, the faster it will leak out. This means you will have to remove and clean the excess grease every now and then, or it will build up and collect harmful dirt. Some guys have found ways to seal the diff, so that its a one time fix, but you will have to experiment with yours and see if thats something you want to do. I would suggest opening the differential and cleaning it out very good. Then reassemble, but before you close it up pack it about 3/4 of the way full of heavy grease. This should give you much better traction for several runs, and if you use a thick grease it will not leak out right away.