Got a 99 ram 1500 has #3 cylinder misfiring, done changed the plugs and wires and it still does it but only on the last gear any ideas what is still causing it?

often worn valves on #3 do that or weak valve springs. Test for that first, and also check for blowing head gaskets which start to blow at one cylinder first

Another question too about my 99 ram 1500, when im driving down the road with an empty trailer or loaded trailer, my overdrive wants to turn on/off by itself quiet often but when I don't have a trailer hooked up to it it doesnt act up like that what could cause this?