Chances are if your getting park and neutral in the right places, your getting reverse also. Check your fluid level again, the last time I had the torque converter out and emptied it, I was adding oil forever. I kept looking under the XJ to make sure it wasn´t leaking out the bottom. I´d run it for awhile, then add, the run, then add, seemed like forever.
I was thinking (nothing really to base my thoughts on, but instinct) that low pressure may be enough to operate the forward gears, but not enough to get reverse to engauge. Low pressure can come from the pump, the TV cable adjustment, a plugged or paritally blocked cooler line, a partial valve body or solenoid blockage. Low pressure may cause first to slip a bit and the other gears to shift late. First and reverse and even I believe some other gears share some brakes and clutches with reverse. Doesn´t seem likely just reverse would go out or act up.
I sure don´t know the AW4 like I do some other trannies, I´ve rarley had to work on one.
Check the cable running from your TB to the tranny. If the cable is really loose it will cause low pressures. Most times when I´ve had reverse go out of a tranny there was a pile of bits and pieces in the pan.
Junk in the valve body is always a possibility. When all else has failed I have a make or break, last resort trick, that may dislodge junk from the valve body, but then again may break your tranny for good.