StrechM said:Unfortunately there is no inexpensive solution. I've used Kipster's diff cups with RRP rings, 3mm screws. They seem to work without breaking. Some say that shimming stock diffs correctly will also do the job.
Kipster
http://www.kippster-racing.com
http://www.kippster-racing.com/Instructions/
There is always UE
http://www.unlimitedengineering.com/Instructions/7.5Diff/7.5DiffInstructions-WEB.pdf
Then there is Maximizer
https://www.ssl-stormerhobbies.com/cgi-bin/seekpart.pl?pn=MPI2321
There are also fast lane machines diff cups. They are only 6061, but they are half the cost of maximizers or kippsters. They are on the bottom of this page: http://www.2coolrc.com/diff_case.html You can buy the diff cup only for $15 directly from http://www.fastlanemachine.com/ for $15.