Transmission With Reverse And Second Only

I Have a 1995 GMC Pickup that only drives in second gear and reverse. It will shift normally when I disconnect the battery for a bit. It then will gradually stop shifting correctly. Some strange symtoms are: the speedometer and tack will fluctuate; the gear selector light will go on or off seemingly randomly; changed the main computer for the third time making sure the numbers match. I've been told by a GM tech that the truck will go to "SAFE MODE". That is, drive in second and reverse only if failure is happening in the trani. Any Ideas would be appreciated. Thanks