So I am not a transmission guy, but was wondering it is would be possible to change a single gear in my transmission with a custom made one?

 

No, it is not possible. Gears are made in sets. If you change only one the others will not fit to it and you won't be able to assemble the transmission.

 

So would I just need to replace all the gears and it would work?

 

What are you trying to do?

 

Preferably keep my first gear and third gear, but with fourth I would like to make it around a .45 and with second a 1.8. I will keep my second gear a 1.50, I really just want a truck that can cruise at low RPMs at higher speeds.

 

Put in some 3.08 gears in the rear hahaha that would do the trick, but you would have to change the front and you would loose alot of low end torque. My truck has 3.31 gears and it does pritty good low end and tacks about 1800rpms at 70 mph ive gotton 21 mpg befor so i cant complain. Pulls a 4,000 pund camper just fine also.

 

Nope, you can't modify your transmission. Get a Gear Vendors Overdrive.

http://www.gearvendors.com/

I don't think you can get 3.08's in 8.8 reverse cut for the front.

 

Well I want to get 4.10s or 4.30s and the gear vendors unit doesn't really help me at all. It costs a ton of money just to get me an extra 50-100RPMs lower in fourth gears.

 

Just because the RPM's are lower doesn't mean you will get better gas mileage - it's going to be downshifting and the TC will be unlocking a lot more.

 

If you think THAT costs a lot of money, try designing and building a custom gearset. That will set you back well into six figures.