i done exactly what you're asking. i just upgraded the gears in the transmission as they went out. all you really need is the metal idler, which is cheap. i think there are 2 though, a aluminum one and a steel one. i went with the cheapest one. it was only about 5 bucks. the differential assembly i think i ordered together. i've only snapped a axle on my stampede one time. this was me goofing off before i set the esc drag brake. i had it off the ground and went from full throttle to reverse, snap...... i replaced it with stock again and have not had any issues, i bash the crap out of it.
however, 2 things. 1. i have my slipper as tight as it will go, the stock transmission prob. would have lasted a lot longer than it did if i had adjusted the slipper.
2. i don't do full throttle when i jump it. if you're on the throttle while it's in the air and land. it's prob. going to snap the axles.
aluminum idler
http://www.toysonics.com/traxxas-1996x-aluminum-30t-idler-gear.html
or you can get the steel, which is prob. what i did
http://www.horizonhobby.com/Products/Default.aspx?ProdID=TRA3696
i also purchased bearings for the above gear, and anything in the transmission that was bushings i replaced with bearings. i can't remember now if the base model i had had bushings or bearings.
http://www3.towerhobbies.com/cgi-bin/wti0095p?FVPROFIL=++&FVSEARCH=2381x&search=Go
theres the differential..
so for under 10 bucks you can have all upgraded metal gears, but keep in mind you may also need to purchase bearings..
i hope i get a thank you now, i spent the last 30 minutes or so searching for that differential and that steel gear lol