I've got a '96 F150 xlt 4x4. Sunday I lost 3rd and 4th gears but 1st, 2nd, overdrive and reverse are fine. When I shift into 3rd or 4th the truck acts like it's in neutral. I'm pretty good with engines. But transmissions is not my strength at all. Where do I begin looking? It seems like if the transmission was shot nothing would work. It was running great until that happened.

 

3rd and 4th are likely on one shaft and the others on the other shaft. You have "lost" those gears, either a shaft has broken or the sliding gear sets are no longer sliding
Now is a chance to see how the transmission works, in fixing this one or you can replace it.

Might need some snap ring pliers and a clean spot to do it. Pretty straightforward.

 

If the transmission has an inspection cover Id start there..In most transmissions this would be the problem area and sometimes accessible from the cab but youll have to remove the seat and the rug to open up the main hole....








Like stated before me its pretty straight forwards...Once you pull the cover the problem will be pretty evident//

If you lack the skills to do such a task its usually a grand to refurbish one and about $700 for a junkyard unit.....

I just got done rebuilding the 5 speed plus transfer case also the rearend and IFS in my 2002 Ranger project...

It wasnt too hard but it wasnt easy either as a 12 ton press comes in handy for doing a majority of the tasks & to save time and machine shop fees so I bought one & I also bought a transmission jack from harbor freight which made doing everything myself a breeze............LOL

The transmission jack would definately be the way to go to get the job done if you dont want to disassemble the interior......

Good Luck