jeep cherokee transmission problem?

Hi there.
I am hoping you can help me out here,my old trusty jeep has recently started jumping out of gear and making a horrible grating noise,the only way i can describe it is its like driving a manual car and trying to change gear without dipping the clutch,once it does this it will do it in all gears except 4lo then after a while i will try it normally again(2wd)and it will be fine,the accelerations there and its perfect in all gears,now this might stay this way for days or it could last only 5 mins,any help with this would be great as i would like an idea of whats wrong before it goes into garage for repairs and hopefully me knowing whats wrong might save me a few quid.
Kind regards
Brian
(hull England)

ps i love my jeep