I need your advice:

I need to change my '85 el 7.5 posi axle gear ratio from 2.41 to 3.73.
I have a spare posi axle so I can do it on the bench. (7.5 inch also)
I have gotten quotes from 500$ to 700$. However, I would really like to do it myself just because I've never done one before. I've been told it is best to have a professional do it.
In my do-it-yourself experience I have rebuilt engines and standard trannies and all the other accessory replacement that comes with a hobby car.

So, I'd like your advice as to whether to tackle this myself or ship it out.

Furthermore, if you think I should go for it, can you give me or refer me to any "how to" information?

If you're mechanically inclined enough to rebuild an engine, a rear end is a piece of cake!!!

It will require some special tools and also some manhandling, that's the part the scares a lot of guys off. I'm sure you can find all the info you need to do the job on the internet and in a good shop manual.

Part of the manhandling comes in when removing the pinion flange. You need a tool to hold the flange and a heavy duty impact to loosen the nut. I can do it by myself with a 24" pipe wrench and an appropriate socket and pull handle, but I'm a big guy. Removing the flange will take an appropriate puller too.

After that it's pretty straight forward but you will need some pullers, or have the parts(bearings)separated from the carrier by a machine shop.

Proper measurement can save some time by reducing the number of times you have to check the pattern/reshim the assembly.

Here's a link to a good vendor for parts.

http://www.tomsdifferentials.com/

Thanks for the confidence. I did it myself and it turned out perfect first try. Didn't have to re-shim. I'm in the process of taking the original axle out of the truck and installing the rebuilt one. I'm anxious to see the difference between the stock 2.41 and the new 3.73 performance. A edelbrock fuel injected 350 will be cranking those 3.73's through a 700R4. Thanks for your help.

COOL!!!! 8)

Glad things went well for you!!!

Remember it's recommended to "break in" a NEW ring and pinion. Even if it's a used set take it easy for a while, vary your speed and listen for any whine. If it sounds ok after a while stand on it a few times then you'll be good to go.