If it were mine I would put a quart or two of Lucas or Hi-Tach in it, after draining out the same amount of the regular oil. Then I would drive it until you could not stand it.
In the mean time I would look for a second transmission, as this is almost always cheaper than rebuilding one, especially with a common one like you have.
By the way, what transmission do you have? A 4 speed, or a 5 speed. The 4 speeds can be found for around $100 or less in good quiet working condition. If it is a Spicer 5 speed with the round SAE 12 bolt clutch housing then I have one that I will sell for $75. It has about 85,000 miles, I drove the truck home and parted it out, it's easy shifting with no problems.
If you plan to keep the truck for ever, and just like rebuilding transmissions then do it. For a work truck, or anything less than a full restoration, buy a used transmission and save yourself the money, trouble, and time.
Grigg
