99 F150 shifting funny

Hello, I have a 99 F150 4.6L V8 auto trans, 2WD with 139K miles. It recently started sticking in second gear. It only seems to do this when you first start it up and drive it. It will stay in second gear, and if you let off on the gas it will shift, and then it seems fine. It doesnt do it again til the next time you start up and drive. The fluid is full and doesn't look or smell burned. I can't afford for this truck to be down for long. Any suggestions? Your help is greatly appreciated!