Some heat will sometimes help it break loose.  Otherwise it's just a lot of time with a pipe wrench.

Thanks Drone. I do have a pipe wrench somewhere. If I do get them out, do you know where I can buy some new ones?

I would give Trail Tough a call.  They picked up Hawks old inventory so chances are they will have one. 

Hey all, I use a suction gun to fill the cases with oil. It's cheap and works great. Kinda messy but no more than pouring it in.
 It looks like a grease gun except where the pump on the grease gun is, the suction gun has a hose. You can suck the oil out of the jug and squirt it into the fill plug hole. Then I store it in a garbage sack because gear oil will get on everything. Also the sack keeps it clean for next time.
Bob