First let me say that I'm not a heavy duty mechanic by any means but I've done a little work here and there in the past. I recently bought an '04 LJ and I'm determined to do as much of the maintenance as I can. So I started with a tune-up and all fluids. Everything went smoothly till I got to the differentials. (30 front & 44 rear.) I had pulled the covers 3 times and STILL had leaks with the RTV the parts store recommended. Someone here recommended Lubelocker seals so I did some research. Now I'll admit that at $20 + shipping they're definitely on the expensive side but to have a mechanic do it for me was going to cost right at $200. Long story short, I ordered them, spent an hour scraping as much of the RTV as I could off and then installing the seals. It's been 3 days now and they seal perfectly!
So for anyone out there on the fence about spending the $$$ to try these, go for it! That's the best $40 I've spent so far.