Brake light/lockup

so during the summer my rear brakes would lock up on the highway. mind you this was my first time messing with drum brakes and i was prob an idiot and installed them wrong.
on the highway at roughly 65-70mph i'd start getting terrible vibrations. that would quickly progress, and get even worse when i hit the brakes. after stopping i noticed the back drums were smoking.
had it towed since i was 2+ hours from home and ripped it all apart when i got home. everything looked like it was supposed to? i mean obviously the brake was engaged while driving and that's why everything heated up i understand that. just for safe keeping i undid my p-brake from the ends of the axle. hadn't had the problem so reconnected them up and have been good ever since until recently. odd.
the vibrating is back and if i hit my brakes i can feel the rear slightly lock up, and way before the front does. my brake light also comes on when i do so. note that i don't have ABS.
anyone have any idea what's going on? i know it's hard without seeing the inside of the drum but this is stupid. i'm about to make a disc brake conversion if i can't get this figured out.

 

Adjusted wrontg/too tight?

 

are you talking about the internal auto adjuster thing?

 

Yep. I haven't been into these brakes yet, but should be same as other rigs..

Can you spin the drum with your hand? Should feel a slight resistance and that's all.