Axel problem-STUCK in another state-HELP

I had my axel & CV joints replaced 2 months ago in my 1990 Geo Prizm. I was traveling this week. As I went down a hill my car raced from 40 to 55mph. When I got to the bottom of the hill my car would not go forward or backwards. I had my car towed to a transmission place. They said my transmission was bad, but I asked them if they had checked my axel. They called back apologizing. My Driver Side axel was out. They said they could not get the axel back in. They purchased a new NAPA axel & put it in. My car worked fine. Today, Sunday, I jumped in my car to return home (300 mile trip). Again, going down hill my new axel came out.

Why is my axel coming out? What is wrong? Wrong part? Differential transmission fluid problem? What else do I need to tell these people who are suppose to diagnose and fix my car. Please help. I am stuck away from home. Thank you!

sounds like to me that it is the wrong axle or that the retaining clip on the trans side of the axel in not locking it in , i would bet that the clip is not locking in correctly

Thank you for your response. I spoke to the people that replaced my axel. They said it was the right part and that the clip worked fine. They say my transmission must be messed up and want to charge me $400 to find out just what is wrong. It drove fine for a day-no gear change problems and the axel didn't fall out til the 2nd day going down a hill. I think it's installation problems and the clip. I think they are not happy about fixing my cheaper problem since they appeared to be overloaded the other day with jobs where they can get $1200-2500. Is it possible my transmission is kicking my axel out? I don't have any more money to invest in axels and labor.

If I have to buy a new car to get back to SC, can anyone recommend a car that will be as good to me as my '90 Geo has been for the last 11 years?

as much as i hate to say it , get an import like a honda or something , they are good on gas and will run forever if you take care of them , as far as the axle goes ,in the 12+ years i have been working on cars i have never ran into a problem like yours