Why did you do u-joints to begin with? If you have to do them again already they must have been cheap ones.
I have used Rotella, Mobile/Delvac and Chevron/Delo and my 95 liked the Delo the best. I now use Schaeffers 7000 for engine and use the
Diesel Treat for fuel additive.
Rotella starts to break down around 3k miles and should be changed.
Check your up-pipes for leaks and remove/clean the intercooler pipes and boots.
If you think it is your injectors you can pull them and send off to Full Force to have the inspected, flowed and new o-rings. Unless you have money don't buy new ones right now.