Body lift: cheapest, allows bigger tires, better rocker clearance, option to raise fuel tank.
Suspension lift a) tightening existing suspension: bigger tires, better frame and body clearance, may limit suspension travel.
Suspension lift b) changing springs/shackles: scarce and pricy parts, need longer shocks, allows bigger tires, gives better frame and body clearance.

Improved diff/axle minimum clearance will only come from taller tires. None of the above will improve your minimum clearance.

Best to do a combo: tighten / re-arc / use spacers for springs (~2"), Roger's 2" body lift, use taller tires. That will give you better than 4" lift at the rockers.