it's in my 92 2500 2wd, 6.2Diesel powered truck.
I had a rear end grenade, and the transmission followed shortly thereafter.
I had the 4L80 rebuilt, and it has not shifted consistently into OD since.
when I got it back, the tech noted on the computer that the brake switch was keeping it out of lockup, and told me that was the likely candidate for original failure, due to heat buildup. I installed a large plate style cooler downstream of the rad cooler, and replaced the brake switch. the lockup started working, but whenever you drive the truck you get OD, until the first need for drop down, then it downshifts to 3rd, and will not recover until you turn the key off and restart. SOMETIMES the OD will stay on during a drive, but not regularly, and not often.
also, at 55/65 with OD on, it almost immediately drops down to 3rd.

theres a 4l80 forum in the 6.5 section

How about the TPS, have you had it checked/set lately, it may be out of range or just plain failing.

I posted this in the 4l80 section too...
with the TPS, the transmission was shifting into OD fine before it died. now, since the rebuild, it will not stay in OD. and once it downshifts out, it will not go back.
the shop is trying to discover the issue, but they are not coming up with a solution. they want me to return the truck to them, AGAIN before they do any more research on the problem. I'd like to have a few suggestions for them to check before I return it... especially with the holiday week going on... I doubt they would get started on the truck before monday...

OK,
codes in computer say RPM difference between motor and trans input.... IE, lockup slipping.
the shop claims new converter was used, and that the diesel and gas 4l80 converter are interchangable. true? if so, what could allow slip? pressure problems? bad lockup? help!