Truck runs fine until it shifts into overdrive. RPMs drop in half until you can get back over 2k rpms. Gas mileage is down too 20mpg. Tune up did not help.
Loss of power in O/D on 1995 Nissan Pickup (D21)
You need to have a proper diagnostic inspection. A tune is for maintenance. If you have a loss of power on the freeway, you need to have a computer scan, a fuel pressure test, and exhaust back pressure test just for starters. How well does your trans shift, does it shift well ? Did the fuel and air filter get replaced with the tune-up, because they should have? For the time being , turn off the over drive switch, that should help, and drive 60mph or under until you get the problem really resolved.