2002 Ford E-Series Van

Transmission problem
2002 Ford E-Series Van Automatic 272000 miles

I have a 2002 Ford Econoline Van. It ran good until just the last couple of weeks. It is getting VERY POOR gas mileage. Like 4-5 miles per gallon. When taking off down the road it will be in 1st gear until you get to 35-40 miles per hour and then jerk into the next gear unless you let off the gas. It hardly gets out of 2nd gear at all unless you come down a hill and coast and it will change gears at about 55mph. In order to keep it up to speed limit speed of 55 mph it is whining in whatever gear it seems to be stuck in. I presume its second gear because when you downshift into second it doesn't change anything. I checked the ATF and it was full. Also the gas smell is really strong. The exhaust smells like straight gas. And when starting the van or idling at a stoplight etc. It stalls out sometimes if you don't hold down on the brake and give it gas at the same time. I know it is a high mileage van, but could you give any suggestions on what might be the problem. Thank you very much.