2003 montana automatic won't engage in "D"

My wife has a 2003 Pontiac Montana w/ automatic transmission. It has about 140k...we bought it w/ 85k in May 2005. All of the sudden tonight when put into drive the van would not engage in "D" it acted like it was put in neutral instead. Reverse along with 2nd, 3rd, and 4th were fine. I was afraid to have her do anything drastic. Instructed her to manually put van in 1st gear and see how it would behave. It took off with a little shudder. We progressed through all the gears while going down the highway getting up to 55 mph fine. However, when coming to a stop and then starting again it seemed like the car was put in neutral when it was actually still in drive. It seems like there is something wrong with first gear. Doesn't seem like the transmission is out however there is definitely something wrong. Fluid level looks very good...jacked it up when we got home and there are no leaks. Shift solenoid problems cause the vehicle not to shift from gear to gear so this should not be the problem. Could this be the the torque converter clutch.
Haven't personally serviced any trannys besides fluid and filter changes. Any ideas?