what exactly are you asking? if you want a locking differential you need to buy or borrow one from another car e30. you could buy them new, but that would be pretty costly. the stock e30 LSD (both for M3's and non-M's) has about a 20% lock; however, you can buy custom diffs with up to around 60%. welding your diff would give you a 100% lock, which would be awful for a car that is meant to do more than just go in a straight line.