1991 Isuzu Pickup 2.6L Auto - Transmission / Overdrive Problems

1991 Isuzu Pickup
2.6L FI - 4ZE1
4 speed Automatic

Problem: No 4th gear (overdrive)

ok, so far:

i've posted on isuzupup.com, but so far, i haven't been able to solve it

http://www.isuzupup.com/forum/viewtopic.php?t=435&highlight=overdrive

from that i've heard it could be an overdrive temperature sensor, but i can't find it on the intake (where it's supposed to be), only the dash temp sensor gauge
apparently it's a somewhat common problem on isuzu's, but it's not helping me
problem: my intake (1991) doesn't match the one in the 4x4wire.com picture

http://www.4x4wire.com/forums/showflat.php?Cat=&Number=647044&page=&view=&sb=5&o=&fpart=1&vc=1

also checked some of the vacuum lines, but they all seem to be correct

seems like an electrical problem to me, but i'm afraid it could be the transmission
i paid $280 for the truck, getting another trans would be like $300
and i can't use my truck for long distance driving, being limited to around 45 mph, w/out high-revs that drain my gas tank

anyone have access to tech information (wiring diagrams, vacuum diagrams) that i can use?
or does anyone know where the sensor is or how i can find it?
has anyone had to deal with this problem?

Have you figured out the issue? I am having the same problem.