I have a 1999 durango 4x4 slt. I was driving it just fine and it started slipping out of gear. Now the check engine light is on and any gear i put it in R,D,2,1, its as if its in neutral.

Start by getting the code, you can turn the ignition key fast 3 times from of to on and leave it on on and the codes will appear in the dash where the odo is. Then you can go to this site to see what they mean: http://www.wjjeeps.com/faultcodes.htm it sounds like a trany problem, have checked your fluid or changed it out along with the trany filter ?

The tranny fluid is good with a good color

Then get the code and take it from there.

My guess would be neutral safety switch or shift solenoids have gone out. Best bet is to read the codes. They will at least give you a starting point.