i want to put a limited slip rear end in my 91 ford f150. right now i have a open rear end with a 3.55 gear ratio To get the parts i need im going to use a nother truck with a 3.55 lsd rear end. Is it hard to change from a open to a lsd rear end also what parts do i need to switch out.

 

Are you going to just swap carriers and use the old pinion with the new ring gear? That won't work, at least not for very long. As gears wear, the patterns wear into each other. Put a used ring gear with a different used pinion gear and they'll fail in short order. The more mileage on the gears, the sooner and probably more catastrophic the failures will be.

So you either have to swap the ring gear from one carrier to the other, at which point you are going to have to start from scratch as far as setting up the gears go. This is one of those things that if you have to ask about it, you should pay someone to do it for you. Gear setups generally run a couple hundred bucks an axle and unless the gears are in good shape the tech may not like the condition of your used gears and won't warranty his work if you don't buy new gears.

The other option is to swap the pinion gear, but you will need to go through the same gear setup process.

Are you going to re-build the LS clutch packs or throw them in as-is? If you don't rebuild them and add some clutches you are going to be very disappointed with the performance of your new limited slip.

Why do you need a limited slip?

 

I have done this a few times and all you need is a tube of RTV & LS oil.
Use your old rig gear and put the shims back in the same places as they were when you found them.
You will not have to set anything up as shims are for the housing and not the gears or bearings.
You may want to replace the clutches if they are worn or the initial breakaway torque is less than 20 lb-ft at the wheel with a torque wrench.