problem with brakes '83 K10 14bff

[nikwho]

Sorry for the double post mods, can you delete the other post in the 73-87 section. I was not getting any responses there.

I have an '83 K10 with 3/4 ton axles swapped under it. Its got a corporate 10 bolt in the front and a 14 bolt full floater in the rear w/ drum brakes. I was told that the axles were from a "comparable year" 3/4 ton truck. Truck was as is when I bought it. I am assuming that it has the stock K10 proportioning valve. My problem is that my rear brakes will lock up very easily with what feels like very little if no braking to the front. has, a couple of times, stalled the engine while braking in gear traveling at 20-30mph. Even with just the slightest push of the brake pedal. Makes for some interesting driving characteristics! Its a nightmare(ok, sometimes kind of fun) in the snow! Any thoughts? I was thinking that maybe its a proportioning valve issue. Thanks for the help guys
Nik