I just had a similar issue. My 99 Jeep Wrangler began to sound weak when starting it. After a few weeks, it wouldn't start. A jump didn't start it either, which I thought was strange. The battery was 8 years old, so I assumed it was the battery and replaced it.
The Jeep started right up...however...
A couple days after I replaced the battery, it began to sound weak again.
My mechanic indicated the alternator was fine, battery was fine and the starter was weak, but not dead yet. The mechanic was not certain the starter was the issue at first.
I asked them to replace the starter because at that point it was the most likely culprit.
After they replaced it, they called to let me know that at first they weren't sure that was the issue, however, after replacing it, they said it definitely was the problem.
The Jeep starts up VERY strongly now.
It has been a month and have had no issues at all after replacing the starter.