I can't comment on the previous work done, but $300 labor for a starter sounds steep. A few days to diagnose the problem sounds even more ridiculous. Take a knowledgable person with you to talk to this guy.

Sorry I didn't (and still don't) have much time to look into this in depth. From the sounds of it there is a very strong possibility that your truck was indeed in need of a lot of work. When someone says "My car/truck has always been great and never cost me a ,lot of money" in a sense they are in fact addmitting it's going to cost more money real soon because cars and trucks are not appliances, they are machines that require regular service and repair. Could a transmission problem and a dfferential problem both exist on a 1990 vintage vehicle, absolutely. Could a problem with the fuel injection system (otherwise referred to as the brain by the OP) also exist on the same vehicle, again, absolutely. The reality could easily be your not getting taken by the shop. The shop may actually be getting raked over the coals by your truck because the typical approach of any good shop/technician is to attempt to do exactly what needs done, and nothing more. However we are limited by the same senses as everyone else and there are no crystal balls that warn us of every possibility that we will encounter. The shop is quite likely going through the routine of finding an obvious issue, solving it, only to find another problem one sitting right behind the first one, (and I'll stress be equally dissapointed with the fact that they don't have you happily down the road yet). The bottom line is the truck won't be fixed, until it's completely fixed. From the sounds of things right now I suspect the "starter" issue potentially isn't just a starter, but may be associated to a flywheel problem. $300 is way too much for a starter labor unless there are modifications done to the truck that require a lot of additional labor. However it would be enough to remove and reinstall the transmission for access to the flywheel. I could see a situation where the shop may work out a deal to repair, or otherwise warranty a new starter if flywheel damage has damaged the new starter.

All in all, communication is the key here. No one expects every person that owns a car to understand all of the potential service issues related to that vehicle. You need the assistance of someone that genuinely does understand exactly what techs do when they fix a car. That person will be able to see exactly what the shop has been facing and would be capable of seeing if the shop has indeed made any mistakes, or if your truck has simply reached a point where significant repairs were/are required to return it to service.

If you would like to forward my contact information to the shop, and they are willing to have me help then give them my e-mail address and we can take it from there.

cardoc3@earthlink.net