Alternator Issues

Hi, I have a 91 with 4.0 HO.
About six or seven months ago my Jeep died, I couldn't maintain enough voltage to drive. Took the alt. to get it tested and was told it was fine. It had a two month old battery whichwas not the issue. I replaced the computer on the firewall from a second working XJ and same thing, no progress made. Finally got a voltmeter on there and found the alt. was not putting out the correct voltage. Unfortunately, the store whom tested the alternator and who's machine said the alt was still good was the only store i had access to and had to buy their alternator. Replaced and everything was fine.
Fast forward to yeserday- I'm driving to work-which is 95% highway and by the time I get there I have 11 volts- both from my analog dash gauge and my digital gauge on my stereo. Driving home last night i was in the 10V range the entire trip, occasionally dipping into high 9's with a CEL. Today i started fine, but voltage was again in the 9's. Dropped my daughter off at school and came out to start up and -DEAD- AAA shows up and jumps me. I live about 3 miles away- by the time I was halfway home the speedo stopped working, stereo flickered on and off and I was buried at 9V. Will not start. Terminals tight and in pretty decent shape (still stock gauge- I know it's on the list) My voltmeter battery was also dead so I couldn't check voltage from alt or battery independently.
My time is severely limited due to work. I don't want to get the alternator off and have the people who warranty the alt. test it and tell me this alt. is fine just like the last one. Any suggestions? Am I wrong in assuming the alt. has crapped out again? Any reason why an alternator would quit so soon? My charging system is not overloaded with amps or 5 sets of aux lighting either.
Sorry for the length of the post. TIA

This is classic symptom of a bad alternator... just make sure all your electrical connections are clean and tight, and that serpentine belt is not loose.

There is a reason products have warranty....

Belt was replaced with new alt. and is tight and very good condition. connections are tight. I'm concerned that their stupid machine is going to once again tell me i have a good alternator like it did last time.

You might want to read thorugh this thread...

http://www.naxja.org/forum/showthread.php?t=933959&highlight=check+the+alternator

I understand how to test the alternator, i'm talking about the store testing it. If their stupid bench tester says it's fine, they will not make the warranty claim regardless of what i tell them. Before I go to the store and get aggrivated with them if they test and say it's fine, I just wanted to see if there were any other real possibilities; I didn't have any that came to mind. I have the repair manual, this section from the FSM and also the what the codes are from the FSM.
Thanks for the ideas and the confirmation that it's the alt. Sometimes I get tunnel vision and close out the idea of another possible culprit.

Good...now if you test it yourself using real life, real time load on it, versus their bench tester, you have something to argue with them about...
If you can get running long enough, drive it down there, and show them in the parking lot that it is failing...then pull it out right there and have them give you a new one.