I have a 1999 Isuzu Trooper. Can't figure out problems

Question

I have a 1999 Isuzu Trooper. 3.5 liter V6. Automatic. My problem is, if I have it in neutral or park, the engine runs great. Smooth, no weird noises, it'll rev up to red line RPMs no problem. However as soon as I put it in drive it loses power. It'll barely drive, won't go over like 1500 RPMs. If I put it in 4 low it will act like it has more power and it will drive better. So I assume the problem is somewhere in the tranny/transfer case/torque converter. Or am I completely wrong? I'm no mechanic.