gear changes
 

Does anyone know how much it would cost me to have the gears in my rear axle installed?

what do you have in it and which way to do you want to go? my 2wd has 3.42's in it and i think they are pretty bitchin. decent fuel mileage and the truck isnt struggling. you figure that you are going to be buying the gears which will probably cost you at least 500-600. plus labor. which could vary shop to shop.

The gears are 3.73 and $180 for them. I'm gonna put them on when I put 31s on it

you have 3.73's now?? if thats the case, i would leave it. you arent going to lower the gear ratio that much i dont think. you will probably end up around 3.42. i know that they make calculators for this sort of thing.
here, check this out. it only takes a second to put the numbers and options in.

If you have 4wd make sure both front and rear have the same ratios.As mentioned 3.42 gears are good 3.73 better.If your going to some serious mudding or pulling you want 4.11`s.

I'm not because I have a 2wd and all I'm gonna pull is a trailer

if I stay with my 3.42 gears will my mpg be effected a lot?

3.42 gears shouldnt affect your milege much,its a good gear ration for hiway driving and not bad for city.if you do more city than hiway then 3.73 would give you better milege.It wont be bad for hiway driving either.I have the 3.42`s in my Jimmy but its 4wd.Its supposed to get 21 hiway and 15 city but I dont know It hasnt been driven in almost 3 years.

What if I put 31s on it with 3.42 gears will my mpg be effected more with stock or 3.73?? And by how much if I stick with stock gears?

With 3.42and 31 inch tires as well as pulling a trailer the truck will struggle to get into third gear and never into fourth. A big strain on the tcc.