Diff swap info on a 1984 f150?

I want to find a ford 9 or 8.8 that has posi from a fox and a little bit of a higher gear ratio in my 84 truck it has 3.08 ratio right now i want at least 3.5 because i want to eventually swap a t5 from a foxbody. Everything in my truck is fine although i think i have some play in my diff. Will i need to change my axles or the complete rear axle. I need to know this because i'm taking the parts to mexico to do the job, i know a really really good mechanic charges less than 5 dollars per hour rate

My baby

Well, the V8 Fox body's topped out at 3.08 gears IIRC. So you're not going to gain anything there.