Yea, E30 318 (4-bolt) rear ends are very weak. I cooked my 4.10LS in my 318is very badly. The internals might as well have exploded. I currently run a big-case 3.73LS.
E30M50, that diff is either a 3.64 or 3.91, they are the only 318 ratios out there, if not 4.10.