There are many different options for picking the right differential. Some of these differentials are too high for highway speeds. Some are too low for fast acceleration. Many of these differentials (bolded) are featured in E30’s. Many of them can be grabbed from E28, E24, E32. Just swap the output flanges and the rear cover from that of your current E30. It’s as simple as that. I do not know the entire differential compatibility between BMW’s in the 80’s-90’s. Not to mention, it’s relatively out of the realm of the M30 swap, so here is a brief overview.
4.10 Doable, but way too high and might lose first gear. Low gas mileage.
3.91 Drivability on the highway will be tough. Compromise top speed
3.73 Revving high in 5th, but have nice pull if downshifting at highway speeds
3.64 In between. If you can get your hands on it, go for it.
3.46 Probably a real good all around gear. Kind of rare to get though
3.25 I had in my M30 turbo. I love it. I cruise around highway at 2500rpm at 70mph
2.93 Better for higher turbo M30’s >12psi