I had a little trail of fluid leaking out of mine at the rubber fill plug, but carquest hooked me up with a new one for just a buck or two. My pinion seal was also leaking but that's a different issue. These covers appeared to have used RTV from the factory; I have no record of the fluids having been changed in mine prior to my ownership, the front was a special female dog to get off, cover was gasketed with some sort of ultra-adhesive orange RTV, had to beat it off with a hard rubber hammer (actually ended up ruining the cheap hammer) and I thought that I was going to have to use a metal hammer and hope for the best.
I had a theory that the hole for the rubber plug for the 35 would be the same size as the threaded NPT plug used on the 30 front cover but after looking more closely it appears that it's iffy if you can thread that hole and get a threaded plug to fit properly. I just used the new rubber plug and it's been dry as a bone since.
This is how I cleaned up my covers (and how I clean up pretty much any under-car or under-hood bit that I remove that doesn't need to be reinstalled immediately)
lube locker gaskets:
yeah, they're expensive but then again I don't even have to bother sourcing new gaskets when I get around to changing the lube again. My front cover was so rusty (probably from road salt/sand/debris wearing off the paint early in the vehicle's life) that it had to cook in the derust tank overnight before I could paint it, but it's back on and leak free now as is the rear. Really unless you see actual damage to the cover and/or you want a HD cover to prevent off road damage or to support the bearing caps (and if you want that, should you really be running a 35?) I'd spend the money on the reusable gaskets, a new fill plug, some primer and paint, and some good synthetic gear lube (I like Redline Heavy Shockproof but there are other good options out there as well) not a replacement cover that you probably don't need.