how much of a lift do you need to clear 33's on an 84 pickup?

 

Probably 3-4" unless you heavily trim the fenders and/or move the front axle forward:
- http://www.4crawler.com/4x4/4R_suspension.shtml#PhaseI

3.5" lift, stock axle location, 33x9.50 on stock 15x6 rims, rub w/o turning:

 

Looking at my 82, I can fit 33s as it sits stock with some trimming.

The real question is how much trimming do you need.
I'd say you could do it with 3".

 

The '83 and earlier trucks have much larger wheel well openings. 33s fit with a lot less lift on them. 31s will rub on an 84/85 w/ stock springs, but I think those fit fine on an '83 or earlier. A lot depends on the wheel width and backspacing. A buddy of mine had rubbing w/ 31x10.50s on his '85, but he did have aftermarket 15x8s (I think that was the width). A 1" longer front spring shackle up front gave enough lift to clear the tires.

 

They do. I'm sitting on some 31s right now.

 

The suspension on my 85 4runner is complete stock except for a 5.5" shackle in the back to level out the saggy rear. I have 255/85 (33.3"x10") interco trxus m/t. I had to pound in the pinch weld on the back side of the front fender. No issues with the tires rubbing. I don?t think the rear shackle helps clear the larger tires. I doubt you would have any problems stock.