Rear end gear replacement

[dbenamati]

I posted a few months ago about my truck and its very slow speed and improvements I can make to move a little faster without replacing the whole axle and transmission. A guy told me tday that I can simply pull off the differential cover, pull the axles apart and replace the gears to what I want or find in the wrecking yard. Is it that simple? My truck is a 68 3/4 ton and tops out at a whopping 55 mpg. In theory, can I find a rear end gear from a different truck and replace the one I have without getting in to a whole new project. I have never tried this before and whould have someone in a garage do it but I have to know what Im talking about first. Thanks,