Car jerks when switching gears

I have a 99 Grand Prix and after I drive it for a while and it warms up When it goes in to 2nd gear or third the car kinda jerks, it doesent feel like its a slip but it jerks is the best way to explain it. It has 102,000 miles on it. Is there some kind of sensor? I check the fluid it is right where it suppose to be. No check engine light is on it just started this yesterday any and all suggestions would be great before I decide to take it to a shop. I plan on going about 800 miles for Thanksgiving I dont wanna have a problem. Please Help

I'd get it scanned with a trans capable scanner, as the problem could be a bad shift solenoid or pressure control solenoid.

Ok I went and put it on a scanner at the transmission shop and a code 1811 came up that was the only code...Can anyone tell me what that is? He is telling me that my transmission needs to be replaced and its going to cost around $2,300. but wouldnt go into detail on what the problem was.