I have a 94 J30 with 230k+ miles and just recently the transmission stopped engaging reverse. All forward gears work fine, no slipping, hunting etc. Checked the transmission fluid (engine running, tranny in neutral) and it was slightly low, added about a quart and reverse began to engage again then after some toggling between reverse and drive it stopped engaging. Did some searching and saw some people with similar problems but no definitive fixes, just wondering if anyone has encountered this and solved the issue. Thanks.

Sounds like you need to refer to the FSM to find out the possible fixes!

Find the entire collection of Nissan and Infiniti FSM's in the link below!
http://www.nicoclub.com/archives/use-ni ... abase.html

Do you have any codes or is the a/t light on? If you have codes fix those first.
When was the last time the transmission was serviced? I would perform a complete fluid change. Before you start get a pan gasket. Check the capacity requirements and get that much oil plus 3 qts. It will probably be about 12 qts and cost about $60. I use castrol transmax import. Here is the disclaimer: there is no guarantee this will work and could cost you $100 for nothing. The alternative is take it to a shop and have them diagnose which will cost you for the diagnosis plus they will probably recommend a flush anyway. DONT LET THEM DO A CHEMICAL OR BACK FLUSH! There are machines that will do what I'm about to explain but it's very easy to do yourself and is the proper way to change all the fluid at once.

1. Get a container that will hold about 3 qts and will fit under the car when on ramps. Mark the container at the 1,2,3 qt level.

2. Pull the drain plug and drain the oil out and Mark how much came out.
3. I don't usually pull the pan and clean the filter but in your case I would. Look for metal in the pan.
4. Pull the filter off. I have never had the filter on a j off but as I understand it is just a metal screen that can be cleaned or you can just get a new one. Most of the nissans I have pulled just have a screen.
5. Clean the pan and screen then replace and torque bolts according to the fsm.
6. Fill the transmission with as much fluid as you took out.
7. Here is where you need a helper
8. Find the output line going to the transmission cooler. If you can trace the lines back to the transmission it will usually be the one entering at the lowest point.
9. Remove the output line from the cooler. Have your graduated container ready and have a helper start the car. Run 3 qts of oil out then shut off. Refill with 3 qts. Repeat 2 more times. You should now have all old oil out of the transmission.

If you have fixed any codes and the flush didn't fix the problem then the issue is likely internal and require rebuild/repair.

Its likely finished and will need overhaul, saying that, doing the above procedure is worth a shot, also this stuff is magic if your issue is a hardened lip seal.
I am a certified mechanic and use this stuff in my personal cars with great success,especially ones with "morning sickness"(hard seals) it will not fix burnt clutches but honestly if its going to work its about instant.

http://www.brooklyn300.com/sites/defaul ... rans-x.png