[Infinitrium]

My stepfather is having a problem with his 1990 Accord EX-R. He replaced the left axle shaft last week with a remanufactured part from Canadian Tire. When we went to install it, we had a bit of a time getting it in, we had to tap on the end of the shaft to get it into place. Now, for some reason, it seems like the shaft is slipping out of the transmission enough that the splines won't mesh with the transmission, it will make a grinding sound. The first time this happened, we just took off the left front wheel, undid the big nut on the outer end of the shaft, beat it back into place with a hammer then buttoned things back up. This was enough to get the car home, but it happened again the next day. We removed the shaft and tried reinstalling it. After adding about 3 quarts of ATF (where did it all go?) everything seemed to be working normally. He took the car into work and it seemed like it was going to work this time, but this problem happened 4 times on his way home, about a 20 minute drive, and of course it happened to be raining right then too. What is causing this? Either something is wrong with the axle shaft, maybe it's too short, or something was damaged inside the transmission when the shaft popped out the first time. Please, if you can think of anything to check, tell me. This is just driving me up the wall.