2000 dodge durango transmission slipping in all gears, added transmission oil drove it around and now it won't go into reverse.

 

oh that's not a good sign, how much fluid did you end up having to add? I bet you do not have low gear either as low/reverse are same band, if it was so low as to starve the system it more than likely did catastrophic damage.
If I were you I would drop the tranny pan and look to see how much shavings/clutch pieces, etc are in the bottom as well as sucked into the filter.