Sirquack, I've been turning wrenches for a long damn time, and about 13 years were spent almost exclusively with Isuzu at a couple of dealers. I can tell you that if the TCC test, accurately described by Jakewash, doesn't get you any results, I'd pay some attention to the O2 sensor. It's located where the left and right bank exhaust pipes come together, and if I had to bet, there's probably a lot of oil on the connector, probably leaking from the valve cover gaskets. Unplug it and clean that connection with some spray contact cleaner, let it drip dry, or blow it out with compressed air. CRC makes a spray called Lectra-motive, works well. Also, if you had any top end engine work done recently, check some ECM grounds to make sure they're tight. You can see one on the front of the intake plenum, secured by a 6mm bolt that points toward the rear of the vehicle. There's also one behind the coil pack, secured by one of the big fasteners holding down the plenum.
I'd also recommend changing that O2 sensor if it's never been done.
Oh yeah, it wouldn't hurt to do a trans service, but it's not as easy as most, because the exhaust is in the way. Also, the gaskets are paper, and they have been holding hands with the trans case for a long time, so you have a heck of a date with a scraper if you do it yourself. There's 2 pans, the filter is beneath the larger of the two. I'd definitely pull the pan down first. If you haven't serviced your trans in a long time, then just flush it, some debris can end up in the valve body and give you some real shifting issues.
Anything I can do to help, let me know. I'll be standing here, holding my beer.
