Hi I drive an '89 xj. While my jeep is in neutral it idles fine, first and second gear also drive fine and run like they always have, but once I get into third and fourth gear and sometimes fifth my jeep starts to shake and it seems like I don't have as much power as I did before. I would really appreciate it if someone could help me out a little
-chase

 

Sounds like you may have lost a driveshaft weight. Crawl under and inspect the outside of the rear driveshaft for evidence. The faster it spins, the more you would notice the vibration/shakes.

 

Also could be misfiring in the higher gears due to breakdown in the secondary ignition components.

Those include plugs, wires, cap, and rotor. ever replaced them?

 

A wheel, or wheels out of balance will shake at certain speeds. Sometimes in a window. "it's cool till 40, then clears up past 45" A lesser problem might shake at 55...