1995 Ford F150 5 Trans. Problem

....driving down the road. After shifting from 2nd to 3rd gear I released the clutch stepped on the gas and nothing. No acceleration, the enging just revved. The samething in 4th gear. No sound of breakage, grinding etc.. However it works in 1st, 2nd, 5th and reverse. Whats up??? How do I correct this problem? Thanks Jerry

maybe your selector forks aren't engaging the gears, or the gears aren't locking on the shaft, so they spin freely.

have a gearbox exprt look at it, but i think that's most likely what it is.