The canister you mention is the Evaporative Emission Control System Charcol Canister. Functions as described above. The lines do have to be connected in the correct fashion, or you will have problems. I know detailed instructions for checking it are listed in the begining of chapter 4 (Driveability and Emissions) of the Chilton manual for 86-99 LeSabres.
As for the transmission. The reason your acceleration was "smooth and powerful" when you shifted the transmission from OD to D is not indicative of a transmission problem. The fact is, you were going 55 MPH, so the transmission was likely already in OD. When you step down on the accelerator hard in OD, you cause a "detent shift" causing your transmission to downshift to D to give you more power.
This is why you tow a trailer in D, to keep you transmission to shifting rapidly between D and OD because the transmission doesn't have the power to pull the trailer in OD but wants to. By shifting to D and forcing the transmission to remain in D, you simply increased your engine RPM to transmission output, thereby increasing the amount of power/torque generated by the transmission and given to the wheels.
"Detent shift" is most noticeable if while driving at 55-65MPH you press the accelerator down more than half way. At that speed, a reading from the TPS (Trottle Position Sensor) of 50+% will cause a (4-2 detent shift." E.g. Your transmission will shift from OD to 2. There will be a momentary pause, then you'll hear your engine rev hard to 4000 RPM and there will be the sudden jerk from the added power as your car accelerates very quickly.
Also. I believe what creslevi replaced is the EGR Valve... will be a round unit with one steel tube leading from the exaust manifold and another steel tube leading to the intake manifold. Mounted on the front of the engine, near the valve cover, lower than the intake manifold, higher than the exaust manifold. Let me know if I'm correct on that Rick.