This is only the converter lockup you are feeling NOT a gear change - it will stay in 3rd and NEVER go any higher unless you shift it to D. I've talked to several Isuzu mechanics and engineers and drove 3 different VXes this weekend (SCed '99, '00 and an '01)- it's ain't a shift! But the locking and unlocking of the converter does feel very similar.

What does this mean:
(L)
The tranny will definitely shift from 'locked' 3rd to '4' (or 'L').
Yes, the tranny will shift all the way down to 1st while in 3rd or 2nd.

Did you perform the rev limt test I suggested?

Try shifting the lever to D after this mysterious shift has occured while you are running at 4.5-5Kand you feel the real shift to 4(D).

I ran in third to almost a hundred (and kept the accelerator floored) up a very steep incline (2222 towards 620) and never had it auto shift to 4th.