FIPG all the way, no gasket needed. Toyota spec for those covers is the Loctite 518 or the Toyota FIPG equivalent — that's what the FSM calls for and it's what I've used every time. The orange Permatex (Hondabond or similar anaerobic flange sealant) is also fine. Black RTV is what I'd avoid; it doesn't like being exposed to gear oil long term and tends to weep eventually.
The one thing that makes or breaks it is surface prep. Both mating surfaces need to be clean metal — zero old gasket material, zero oil film, zero grease. I use a razor blade to get the bulk off and then wipe everything down with brake cleaner and a lint-free rag. Don't skip the brake cleaner step, even if it looks clean. Oil contamination will compromise the seal.
Apply a thin, consistent bead — about 2-3mm, running just inside the bolt holes, not outside them. Seat it and run the bolts finger tight, then let it tack up for 10-15 minutes before torqueing. That waiting step matters because FIPG needs slight compression as it starts to cure to seal correctly. Torque in the correct pattern, fill the diff, and don't be tempted to drive it for a few hours.
Done it this way probably a dozen times on various 4WDs and never had one come back.
