The splines on the new cv shaft should slide into the diff without too much effort, but the snap ring can be tricky. Don't slam the shaft in, but try moving it slightly back-and-forth or up-and-down as you push it in. After it's seated properly (be sure it's all the way in so the snap ring retains the shaft within the differential housing), then proceed to re-insert the outer part of the cv shaft into the hub.
I've done this more times than I can count over the years (stupid remans, and stupidly high dealer prices here in the boons). I've never had a problem or damaged anything--just be patient. Good luck!

You're almost done...
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1990 5spd V6 Camry (Still kicking at 393,000km) 1991 Celica GTS -- Pappa needs a 3SGTE...and AWD for all this friggin' snow
Honda my A$$, you just can't kill a Yota...