I know this may sound like a stupid question but I recently purchased this truck and would like to swap my wheels for 15's...but have no clue how or where to begin in regards to lowering her.

Other cars I've owned you simply swapped everything out with some H&R's or Eibach springs and Sport shocks.  But with an old truck....

Can anyone point me in the right direction?  Looking for like a 4" front and 3" in the rear kind of set-up.

Thanks.  

you can crank the torsions and put some blocks in it. they don't make drop spindles for these years, but you can modify the lower arms to fit the 84-88 yota ball joints then run a 2" drop spindles from those years.