Get a scanner that can read any "pending" codes. These do not always trigger the CEL.

My 97 F150 had the exact sysmptoms you describe. Under light load at approx 40-45 the transmission went to bucking and got worse over time.

Only with the scanner was I able to get to the root cause. A cyl 3 misfire. Turned out to be the plug wire. 5$ fix.