03 zr-2 over heating

[truckdude239]

hey guys friend of mine has a 03 s-10 zr-2 with the 4.3. started having issue overheating . It only over heats on the high way or pulling a trailer. rest of the time it is normal (for the most part) Heres the kicker parts replaced: rad, water pump, tstat, rad cap,fan clutch, flushed system, refilled antifreeze with peak life time coolant. and it still over heats. He took it to a shop, and they told him sorry cant figure it out. there is no water in the oil intake isn't leaking. so we are at a lost to figure it out. had another shop say transmission was probably on the way out and building to much heat and the rad can't over it all. i don't believe this one we changed the tranny fluid 10k ago and it still clear not smell of metal in it. soo any ideas where to look next?. was thinking maybe the lower rad hose sucking shut under load maybe?

any help is helpful