Trannie sucks
 

I got a '91 f150 w/302ci 5speed w/overdrive, and i cant **** for crap!! 1st 2nd 3rd 4th and 5th gears grind all the time, 4th and 5th dont grind as much, i know that the fluid wasent changed like it should (by the people i bought it from), called the local dealership and was told it was a mazda 5? any ideas what could be wrong???

any and all help would be great.

you probably have a lot of dirt and grit build up in there. If you havr the time and know how i'd suggest removing the shifter and cleaning it out. We don't like crunchy shifts ;)

Sold my '92 with 90K on it and the transmission was better than my 2000 5spd.
Every Gear???? - dump the fluid - have a look at your clutch - sounds like it's not there anymore

I sold a 91 F-150 with the manual 5 speed several months ago. I had the same problem you do on several occasions. Does the shifter feel lose? A lot of free play in it ? If so pull the rubber boot and check the top cover bolts that secure the shifter to the top of the trans. Mine came loose twice causing shifting problems. The bolts are metric threads with a Torx head PITA.
The guy who bought the truck from me finally tapped the case so standard bolts would fit in it. He went so far as to Loc-tite the bolts in.

GA-V-10