I have a 99 Durango slt 5.9L 4x4 231HD case, I believe it has a 46re trans but am not sure of that. I have no reverse but all drive gears appear to work fine. I can feel the trans "engage" in reverse but when I hit the gas it just revs. I'm not even sure where to start. Fluid levels are ok and the fluid looks fine. It has been leaking slightly, I have to replenish a pint about every 3 months or so. It has had some mild slipping but nothing that I would say is abnormal for a 145k truck.

 

It could be a number of things cause just the reverse to fail... First i would start by dropping the pan and making sure your rear band is adjusted correctly. If so and the problem is still there, then i would check your rear servo. If all looks good then your looking at a overly worn rear band or rear clutches. Honestly, If you have to tear it down to the point of replacing a band for a clutch pack you would be better off just having a full rebuild done..

Just for FYI, i had the same issue around this time last year. I was able to drive it for 4 months till i got the time and money to have it fixed. It ended up being the rear clutches were overly worn and burnt together. I ended up doing a full rebuild myself in my garage. There is a sticky here on the thread i wrote up about it.