The round thing (if I know what your talking about) may or may not push all the way in. If it hits inside the tranny and there is still some round portion left, that's as far as it goes. You need there to be some sliding for and aft of the splined round portion and the output shaft. That way when you hit a bump with the rear wheels and the shaft moves for and aft, you don't pull the shaft all the way out (usually unlikely) or bottom out in the tranny and wreck it's innards. There is typically about an inch of the round portion left outside of the tranny with about a half inch of travel before it hits inside the tranny. If you don't have about an inch then your yoke is hitting something inside the tranny like a bearing with too small of an ID, or the grease inside the splined portion is keeping the air from escaping and creating an air pocket. This should yield a spongy feel instead of a hard hit when it stops going in.
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