"Positraction" is the name of a limited slip differential used in GM products. Ford used the "Traction-Lok" (aka TracLok). But neither "kicks in" - they're ALWAYS in as much as they're going to be. The torque of the engine against the rear axle will cause one side to always have more traction than the other, and after it revs up & the torque drops off, they'll even out. Also, as the spinning tire heats up, its traction increases (on pavement), so the combination of those effects is why you're feeling it suddenly grab after spinning for a moment. Also, you're undoubtedly letting off the gas pedal slightly after it squeals.
The shocks have no effect on braking or traction (from a stop).
Pull the drums & inspect all the hardware. Click my signature link & look in the Brakes & Hubs album for more info. Be sure to check that all three E-brake cables move freely & release fully. There's also a TSB about cheap brake shoes causing aggressive brake response, so look for that. I might have put it in this thread
You can dl a free owner's manual PDF from Ford
for most '95-up trucks, and there's not much difference between a '93 & a '95 F150.