Possibly low fluid.
Is it an automatic or manual if it's an automatic you may have a bad u joints and same for manual, as well as synchronizing gears may cause it to be hard to shift. Otherwise i'd say it's your u-joints
A bad transmission gear can cause your 1999 Nissan frontier to shift hard. Low transmission fluid can also cause the transmission to shift hard.
2006 ion shifts hard
Odds are the clutch is in need of a adjustment or replacement.
i have a 200 f150 p/u if I put my foot into it it will accellerate and shift as it should until 60 mph. at that time it will over rev and I have to slow down and dissengage over drive.
There are a number of conditions that can cause an automatic transmission to slip into protective mode. To prevent damage, if a transmission sensor detects low pressure, slipping or any of a number of conditions, it causes full pressure to go to the shift circuits, making the transmissions shift hard. The only way to determine what's causing the hard shifts is to have a computer diagnostic scan.
check for a stretched/frayed cable
Sounds like the clutch is on it's way out.
The most common reason for an automobile transmission to shift hard is low transmission fluid. Make sure the transmission fluid is at the appropriate level.
Sounds like the shift linkage is broken or disconnected somewhere. You'll need to find and repair the problem. Shouldn't be too hard.
Did you use the right transmission fluid or did you forget to maintain your transmission otherwise, your gears are broken and needed to be replaced.. chances are you need a new transmission.