I had the same problem on my 83 C20. Don't let anybody work on those seals until you/they read up on GM rear wheel seals for floating rear axles (aka "axle seals"). GM changed the design on the rear wheel seals for floating rear axles because the original design is prone to leaking. The new seals are a two piece design, not a one piece design, so it is designed more like a bearing. My rear drums were ruined also, but you should buy the drums and 2 piece seals yourself and have them installed by a shop that wants your business if you can't do it yourself. GM has a bad original one piece design but has not owned up to it.
Here is a quote from another GM post on this issue:
Exclamation Re: 99 Chevy 3500 4x4 rear wheel seal keeps leaking
Had a long chat with Chevrolet today and this is a common problem and they have a special seal for this. the new seal locks onto the seal flange and in the hub then it spins within itself (a 2 peice seal) it is nice that they let me buy a new housing and hub and 7 sets of brake shoes before they finally give me the seal for a cost of $19.98 vs $4271.00 already spent.
From another post - I don't know if this is the correct p/n for your truck, but you could ask your Chevy dealer:
Original P/N 26061024 replaced by 15823962
http://thedieselpageforums.com/tdpfo...d.php?p=237221
Shop around for the drums, shoes, wheel cylinders, etc. and you can get some good prices. If you don't have a good local source, I would try the two AC/Delco online suppliers with free shipping for genuine GM parts at aftermarket prices (usually).