The subframe mounts in E24's and E28's do not live a long happy life under hard use. I ripped brand new mounts apart in less than six months. Even when new, the subframe mounts allow significant movement of the rear subframe. As far as I can tell, there are no aftermarket parts available.
I've heard rumors that people have filled the subframe mounts with polyurethane, but I could not find any first hand information. After a little reasearch and tips from other BMW drivers, I decided to try Devcon brand Flexane 94, a poly that starts life as two separate liquids and ends up as a 90+ durometer poly. At the same time, I also tried reinforcing a new diff mount. I found a 1 lb kit at a local industrial supply house for about $30. After taping up two new bushings and a diff mount with clear packing tape to make my "molds", I mixed and poured. The 1 lb kit was more than enough for my three parts. Ten minutes later, I removed the tape and found some ugly but rigid parts.
I've had the parts in for seven months now (as of 12/99) and they show no signs of cracking. The poly subframe mounts definitely tightened up the rear of the car. There is a down side for those who care about noise - LOTS of noise. I can now hear everything from ring and pinion lash to driveshaft u-joint slop.
With a new driveshaft (4/00) most of the noise went away. Ed 6/3/00
A few people have asked me for a source of liquid polyurethane. I have since been sending people to McMaster-Carr. People have used both the 80 durometer and 94 durometer poly with good results. The 80 durometer is a little softer and may transmit a little less noise. The 80 durometer 1lb poly kit, #8644K11, and 94 durometer 1 lb kit, #8644K18, are each less than $20. You can even order online with a credit card.
Ron Nixon sent me a couple drawings of the tools and methods he used to replace his subframe bushings. I havent tried these. The usual disclaimers apply.