Removal of rear axle help me!

[ShortBedChevy]

I basically need a new axle on the rear end... I was doing a burnout the other day and i must have stripped out the gears... The weather hasnt been good enough for me to take a peak... but i dont have the tools to adjust the ring and pinion if thats all that went out, and the shop is going to cost me a fortune... So i figured i would call the junk yard, and grab a new axle for my rear end... do the gears have to be the same as the front axles? its 4x4...
Also, how hard is it to remove and install a new axle? im 16... poor... and have limited tools, is this a project i should consider? or park the truck until i get the 1200 the shop is looking for? I could get a new axle for about $150, and then i could drain the fluid out put in a new gasket and synthetic oil, then put it in the truck right? is it hard to do? and do i have to aline the driveshaft up? or just make sure its gears are good?

just now thinking about it.. i could have striped the driveshaft huh? the drifshaft moves, but the rear end wont go anywhere...
what should i do!?
If i take the rear cover off... i will lose all the fluid.. if i do that and decide i need to take it to the shop.. could i just put the cover back on, and keep it in 4x4 and drive the few miles to the shop? or would no fluid in there seize it up? HELP!