transmision Problem

My automatic transmision is automatic no more!!!!
I have a 88 jeep Cherokee Laredo, the trany use to work fine but sudenly it stop doing the automatic shifting either up or doing, so I have to shift manually.

Any sugestions will be apreciated

Thanks

check your throtel positon sensor. ill bet my bottom dollar thats the problem. my truck did the same thing. or check you your tranny filter it might be pluged. hope this helps