1999 Dodge Durango won't start?

I had an 2001 Dodge Durango with the 5.9. I had a similar problem that stumped me, I thought it was fuel or the alternator, the only way I could drive it was to jump start it and take the cables off of it with it in drive and go right then or it would die, at a stop I had to put it in neutral and keep it reved or it would die. It ended up being just the battery, simple and easy. I had taken the alternator off and had it tested and everything, it was very weird. My advice would be take out the battery and go have it tested, it is usually free at most part stores. Hope this helps you.