97 Chevy k3500 overdrive transmission problems?

I have a 97 Chevy 3500 dually with 4X4, 350 engine and auto trans. My problem is when driving down the highway in OD truck losses all power and the RPM gauge drops down to 0, when I shift from OD to 3rd the truck runs fine. Does not shift hard and all other gears run fine just the issue with overdrive. I had the power flushed and added conditioner but did not change the filter, changed engine oil and filter, changed all spark plugs, changed thermostate and flushed radiator, changed pcv valve and air filter. Can some one point me into a solution for my problem with the Overdrive issue. I really dont want to replace the whole transmission.