I know this is an old post now, but since the vehicle still moves forwards (in manual low only) and backwards there is only a couple possibilities according to the mental flow chart that was thumped into my brain when I built and troubleshot these about 40 years ago for the rcmp and taxi companies.
First, we know the rear clutch and low/reverse band is still good or it wouldn't move forward at all in manual first gear, so the rear sprague isn't engaging in drive.
The front clutch (and rear band) is still good or it wouldn't back up.
Since the truck doesn't move manually shifted in drive or second, then either the over running clutch is bad (as in spinning in the rear of the case) or possibly (but less likely) a planetary is blown because one is used in forward and the other in reverse.
While dropping the trans pan and valvebody will allow you to inspect the front band, alternately you can loosen the front adjuster nut and turn the external adjuster screw in till it gets tight, which should be no more than 6 full turns max.
If it keeps going till it's flush, then the front band is broken (foregone conclusion).
If it's within spec when tightened, then back it off 3 turns and drop the pan and look for pieces.
If there are none, and there is a hill nearby that you can go high enough up to coast down fast enough to hit drive (about 30 mph) and shift the lever to drive with no gas. If it engages drive solid then both sets of clutches are good.
I just realized that it's possible the passing gear lever could be jammed down too, but it really sounds like one way sprague to me going by my old flow charts. Bolt in replacement is much better than the pressed in spline in aluminum style.
Another thought (that I already dismissed for the reason below): if the trans feels like it's trying to go forward, but it feels like a brake is on, then the park rod could be bent and engaging until the lever is moved all the way to first. That however wouldn't explain it moving in reverse unless it's bent in such a way that the rod comes out of the parking pawl in reverse, then goes back in neutral through second and pops back out in manual first.
I've seen my fair share of weird things with these transmissions. I had the output shaft bearing cage come apart and turn into a one way clutch that even mystified a shop with 50+ years trans experience. A tiny piece of that cage then got caught in the governor and caused the trans to downshift to first gear at 75 mph, tearing the trans case apart and exploding the torque converter.
I even had one brand new lockup converter that was doa (not a problem IH suffers from I hear. I wonder if the last couple years of production had the channels in the case for lockup?
Tom hand's guide to the chrysler torqueflite automatic transmission
Lockup 727 vs non-lockup