6 speed suddenly rough shifting

[1976gmc20]

There is a steep winding hill going down into the west side of town. This morning after about 50 highway miles we turned on the road down the hill and after getting rolling again I pulled the lever down into M which holds it in third gear because that's where it is by then (about 20 mph). I always do this going down this hill.

At the bottom of the hill there is a stop for a traffic light then another block through a 25 mph residential area to a second traffic light. After that it levels out to a main 35 mph city street.

So coming to a stop at the second light I shift back into D. Upon the light turning green and trying to go, the transmission jerks like it wasn't in gear at first. From that point on it seems to be shifting rougher than normal. It isn't really bad but just noticeable because normally you can't even feel it shift just notice the rpm drop.

Transmission fluid is full. Haven't been towing or doing anything unusual lately, just a trip to town and back every couple weeks, and a couple of short drives locally. I had stopped at the little park about a half mile before the turn down the steep hill and it seemed perfectly normal accelerating back out on the highway. Truck only has 76K miles.