Transmission problems... I think

[RW68RSConv]

I have a '72 Jimmy, 4WD, Turbo 350 Transmission. Because of the issue below, I changed fluids in the trans, trans filter, topped off transfer case (was very low), rear diff, etc hoping it was a dirty trans filter or something in the drive train. No change.

Here's my problem: sometimes in the city but always on the highway, cruising along the truck will be running fine then it starts "lugging" - like its being dragged back or towing 10,000 lbs and I can't go over 60, but then - almost like a shift - runs fine for a few hundred yards then back to "lugging". While 'lugging', if I put the truck in neutral it revs fine & instant response, but as soon as its back in gear, it's back to lugging -- so I don't think its the engine.

Tonight, going about 50 it felt like an improperly set 700R4 feels like when shifting up/down trying to find the right gear -- except this was between "lugging" and running correctly, but not actually shifting gears.

Wish I could explain it better. Any ideas?