The 3/4 & 1 ton used the same axle & housing. There are 2 places that could be leaking: the inner seal between the inside of the hub and the differential housing and the floating axle to outer hub. Usually the inner seal is the one that leaks, and the oil shows up in the inside of the wheel/tire. The outer seal on the outside.
Replacing the inner seal is tricky, but not difficult. When you put the seal in the hub, the spring side of the seal goes toward the outside of the hub, which is the oil side. Also, inspect the differential housing for grooves from the seal. If there are any (very likely) you can buy a sleeve to go over the housing. Cost is about $30 for this part. The seal is about $6.
The hard part is putting the hub on the differential without breaking the sill. Truck mechanics have a dolly to hold the drum/hub. I finally gave in last week and made a plywood tool using two 8"x8" pieces of 1/2" plywood with 2 shelving brackets. Cut one piece with a moon in the side and drilled to attach to the wheel lug bolts, formed a right angle, and set it on my platform jack. Set it to the right height and slid the drum/hub on. What happens is when you install it without support and push the drum/hub on and things arent lined up you damage the seal. If you can disassemble the drum from the hub, you can install the hub first, which is easier.
Now for the outer seal. Mine has aluminum gaskets. Clean all the mating surfaces and the bolt holes. I then use ultra-blue rtv and apply a thin layer to all the mating surfaces and the bolt threads. This is also a good time to buy some new grade 8 lockwashers for the bolts, the originals have lost their spring.
