RE: found out problem
OK....one thing at a time.
First, I assume your hub's are not stripped as you mentioned. Second, you found your driveshafts and they are both there. Good. And yes, it is suppose to be loose. Thats fine. You also mentioned it turns the rear wheels. Good. You also mention the problem is not inside, only outside. ? A little confusing but I'm guessing you mean the there doesn't seem to be a problem when the truck is not running and you are pushing the truck around (inside the house). Although, the problem is only when you start the truck and drive it (outside). OK. Hopefully we are on the same page. Lastly, you asked where are the pins. ??????? Don't know what you mean there.
Your next step is to check your rear differential. Do this by holding both front wheels and one of the rear wheels and turn the other rear wheel. When you do this, it should try to turn the other three wheels. The harder you do it, the harder it will try to turn the other wheels. If you do this and you hear and feel some crunching sounds and/or you are able to turn the free wheel without turning the other three wheels, then your differential is toast.
Now, earlier you said you just installed the 4 extra gears (or spider gears), then you later said your LHS did it. I'm guessing your LHS did it. The reason why I ask is because if they did it and your diffs are gone, take it back to them. There is no reason why they stripped out with the spider gear mod. Either they didn't do it or they didn't do it right. Hard to believe but it can happen. Hold them accountable. My next question is did they do it for you because you were having a problem before or did they do it because you wanted the upgrade before having a problem? I ask this because if you had a problem and they used the old diff cup, then it is possible the old worn diff cup caused the spider gears to strip. Again, its hard to believe your LHS would use the old diff cup if it looked like it would cause a problem but who knows. You didn't cheap out and tell them you didn't want to buy a new diff cup did you?
Check your diff and let me know ow you make out.