what do i need to do to swap the ifs in my '94 f-150 4x4 to a solid axel?

Fabtech makes a kit to do just that though its a lil pricey, bout 1800 bucks. Sorce yourself a front axle from a 78-79 bronco (dana 44) And use there kit. Or if you have the skills or help from someone that does it should,nt be to hard to adapt the entire front suspension from your donor. just remember you will probably have to fab some of the brackets. And the 1800 might be worth it to make sure it right  

It really depends on the route you want to take, i know people who have done leaves on their 1/2 tons, one guy did a 4 link coilover setup, and i just snaked all the spring towers, radius arms, and brackets and axle from a '79 and just modified it to fit my frame.  '79 frame is a little narrower than the newer frames so i narrowed the spring towers and moved the radius arm drop brackets in a little to compensate for this.  All told, the swap and lift up front cost about $1500 ($1000 for the rear 9" of height).  

I'm pretty sure you can use a F-350 shackle flip for the front, along with F-350 leafs and front end for fairly cheap.  

I was gonna use the front end from a 350, but to do it "right" id have to cut the first section of frame off, due to the stupid design of the impact riges on 1/2 tons, and weld on the front of a 350/250 with hangars, then fab up some shackles, and so on.  Its easier and faster to do it with coils and 4 link or even easier with radius arms.