Hi Harry
Is your truck leaking around the splines of the propeller shaft flange? I found this leak when running the engine and transmission on the test stand. If you run the transmission in gear without the propeller shaft flange in place the gear oil just pours out on the floor. With the flange in place and the washer and bolt tight it appears that the gear oil can still leak around the splines and leak past the washer and bolt. Was getting more oil on the back of the flange than the front. When the propeller shaft is fitted and bolts tight it seems to trap the oil between the two flanges. Quick dirty fix for this leak was to pack the space on transmission propeller shaft flange before fitting the washer and center bolt with heavy grease.
I was surprised how much gear lube poured out even at idle.
Cheers Phil