1992 F150xlt , 302, E40D transmission.

I don't hardly drive this truck, but the past year it has had a bad vibration at low engine rpms. If i'm in third gear it shakes pretty bad around 30-35mph but once I get past that its fine. If Im in overdrive around 40-45mph, it'll shake, but then when I turn overdrive off and pick the engine rpm's up it'll run smooth. I've replaced the engine oil but no clue when the last transmission change was. Can't really tell in 1st/2nd gear because the engine rpms changes so fast.

Im trying to figure out what needs to be replaced. Is this the famous torque convertor shutter? or do I just need to replace the transmission oil? Is it an engine problem or a transmission problem?

Any help is appreciated