Transmission Flashing Red Light, Non-Shifting Symptoms

The transmission warning red light on my Isuzu Trooper 1995 started flashing and only the "R" and "L" shifts in 2 WD function. All the
other gears in 2WD and 4WD either won't move the car forward or are too sluggish, similar to a disengaged clutch in a manual stickshift transmission. I tried filling up the fluid but it dodn't do any good.
Any thoughts/suggestions before I have it towed away for an almost expensive checkup/repair bill will be appreciated. Thanks!
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