The secret to rebuilding a transfer case so there are no leaks for a long time are as follows. Its been a few years but here goes:
Don't replace the yoke. Get a speedie sleeve. It will be better than a new one. It will have a groove in it, it will leak if you don't sleeve it.
On the inspection cover, hammer the holes flat on an anvil. It wont hurt to dimple them from the inside with the dimple toward the bolt head. Check on a piece of glass with sandpaper taped to it. You will want it as smooth as possible with any dimple facing the bolt head.
On the felt seal, use permatex brush on gasket sealer on the outboard end to keep oil from leaking out.
Chase all holes in the casting. Do not use a tap, use a thread chaser. Taps will wear down the threads.
One the input shaft, use a new bushing and drill a hole through the bushing that matches up with the hole in the outer shaft (Model 18)
On the endplay. If the manual says .008-.012 endplay, shim it to .005 or less. Reason being thermal expansion, wear and run in. Use gasket sealer (thin coat) on both sides of the shims. Use gasket sealer on the mainshaft ends where they enter the case.
Most of this is for a Dana 18 but some is usable for a Dana 20.
I never used silicone sealer for anything. It will fail and leak. Guaranteed.