I didnt see if you had a stick or auto on this post so its a shot in the dark here. I had the same issue with my stick and it was caused by the 4th gear sense switch failing. To remidy the problem I simply shorted the inside of the OD control module across the outter most pins (cant remember the numbers). This gave the module the ground it was seeking and returned OD to normal operations. Mine would fall out of OD and attempts to return it lasted only a few seconds because the switch was bad. I now have a second set of "gears" by using the OD and it amounts to the following:

2+OD=2.75
3+OD=3.75
4+OD= standard OD

Dont engage OD and immediatly resume power, give it a second to build pressure because of the lower driveshaft speeds. 60KM/H is a good starting point and passing power at 3+OD is slightly stronger than normal 4th because of the 0.25 or so reduction.

Badge988