1989 GMC C3500 Rear Brakes leaking oil

It seems the rear brakes are getting oil in the drum from somewhere, either brake fluid or rear end fluid. I've changed the shoes twice in the last year and oil still gets in rendering the rear brakes useless. i always change the seals and use factory parts. This is a 1989 GMC V3500, 4WD crew cab. I have a utility bed on it so it's very heavy. I NEED brakes. Imagine that. When I take off the rear wheels and drums I try to inspect where there is a oil trail, but as of yet I haven't found any. The oil runs out the back side of the drum down the tire, making it visible from under the truck. I also see oil coming out from under the cap at the lug nuts. Any advice would be greatly appreciated. Thanks. Brian