As I recall, the rear band is "low/reverse", i.e. if it wasn't working I think you wouldn't have 1st gear either (check the manual). Anyway, some practical experience that might help. My 96 Voyager 2.4L lost reverse, but all the other gears worked (or at least it went forward). I know you are thinking "mine ain't no minivan", but that A-413 tranny is also a 3-speed "Torqueflite" and essentially the same inside as yours other than the differential section. After swapping in a used tranny (tougher job in a FWD), I tore apart the original for forensic evaluation. I found the only problem was a torn lip seal on one of the servo pistons. I could have easily fixed that from underneath (just need snap-ring pliers). You might not even have to remove the valve body. There is an upgraded piston (<$20) for 904 tranny's (and my A-413) that keeps it from cocking in the bore (tearing the seal). Your problem could be that the rear band just needs adjusting. Either way, remove the pan and see. Get the correct filter. Later ones will starve your rear pump and fail it.
While fixing the servo piston is easy, don't fear a tranny rebuild. After tearing the A-413 down, I decided they weren't as scary as the shop manual suggests. I later had to rebuild the used A-413 (crappy rebuilder job), then the A-604 4-spd tranny in my V-6 minivan. The later was even easier since has no bands (just more clutch plates). The shop manual makes it very easy, with step-by-step drawings. I downloaded the A-413 one free from a Neon racer's site. I recall you can find the A-904 manual too somewhere. If not, ~$20 on ebay or Amazon. I found the ATSG manual is just a copy of the Mopar one and not as thorough.
