I have used both factory and Flaming River. I recently revisited the factory u-joints and got them to to work well. I soaked them in automatic transmission fluid (ATF) overnight and that helped loosen them. Then heated them sparingly with a propane torch and it loosened them more. After a final soaking in ATF they were essentially perfect.
If you already have the factory units maybe give the above a try and save more than $300.