l have a 91 Nissan truck and it is has started having issues in 3rd and 4th gear.All other gears (1st, 2nd, 5th and reverse) it goes into easy but in 3rd and 4th l have to wait for the rpms to go down before it will go into gear without grinding.Any ideas of what it could be?It has plenty of clutch and transmission fluid.
If it is the syncros is there an easy way to replace those?Or do l have to take my whole Transmission out?

Bad internals (synchros).
Just need to rebuild or replace it with a used one.

YOU NEED YOUR TRANS REBUILT THE SYNCHRONIZERS ARE WORN OUT

you have a bent synchronizer. MUST be replaced to stop the grinding noise between 3rd 4th gear. Sounds like someone missed a POWER SHIFT.

Many times the wrong fluid is used & it ruins the syncronizer action. Rear End grease 80wt/90wt GL-5 should never be used in Nissan Manual Gear boxes. 75wt/90wt AP l GL-4 gear lube & synethic 75/90wt GL-4 by Redline oils called MTL may also be used. GL-4 Which does not have the pungent rear end grease smell. Mineral oil it is sometimes called. If the syncros do not respond to several fluid changes. The transmission will have to be removed & completely torn down & all five syncros & shaft bearings should be replaced. Very expensive.