Tranny?

Beginning within the last few days, when in either 3rd, 4th or 5th gear (5 sp. manual) when I quickly accelerate the tranny acts as if it's slipping...the tach revs up suddenly and then the gear seems to kick in. It only happens under hard acceleration.

Any ideas? Suggestions?

My opinion;
Bad clutch... classic symptom when it only happens in the higher gears...
it doesn't slip in the lower gears because the engine is not putting out the power for a given RPM... not as much effort to move the truck.
I don't consider it dangerous, as you can pretty much drive it until it won't transmit power any more...
But I urge you to address it soon.