Here is my $0.02
You have no more than a 20% chance that replacing the solenoid pack will magically fix everything. I wouldn't rule it out but I think it's unlikely. Perhaps in the beginning it might have been a bad solenoid pack, but what concerns me is the report that the transmission fluid was dark and needed to be changed. This isn't normal, I suspect this transmission has been acting up for years, and the previous owners simply ignored it, and during the meantime the transmission has been slowly getting more and more destroyed by the bad whatever it is. It might have been that the seller pulled a stunt like pouring in Dr. Transmission goop into the transmission fluid right before trading it in to the dealer, so that it would work OK long enough to get the sale completed.
The big problem with replacing the solenoid pack is that if it doesen't work, then the only alternative is a complete teardown and rebuild, and a rebuild isn't going to be warrantied unless you put in a virgin rebuilt or new selonoid pack on it. So you are in the situation where you just dumped nearly $300 into a part that now has to be tossed out and replaced, simply because there is a chance that some crap got flushed into it when it was put on the malfunctioning transmission.
If this was my vehicle I would just bite the bullet and have the transmission torn down and rebuilt, and put in a new converter, and solenoid pack, have the cooling lines flushed, basically the works done on it, and make sure the entire job was warrantied good. And yes, I would know that there's a
20% chance that I wouldn't have needed to have done something that radical with it. But here's the deal. If you have a complete overhaul done now, you know it's going to last you a good 80-150K miles. The 3.8L engine is a tank - 75k miles on that engine is like 150K miles on most other engines - it's going to outlast that transmission, even if you have the trans rebuilt right now. And, there's also the safety issue - while I would trust myself if one of my vans suddenly shifted into limp mode while tooling down the highway at 60Mph, I definitely wouldn't trust my wife in that situation. Do you really want to risk this happening to your wife, alone one afternoon on the highway? You might end up with that 99 GV crumpled up in the ditch with the airbags blown, a pretty furious and pissed off wife, and $100 from the local wrecker to tow it off.
Also, my other piece of advice on this. I would be leery of that dealer. From what your saying the dealer sold you a van with a 60 day warranty, and sometime on day 90 they are telling you the transmission fluid was dark and needed to be replaced?!?! I'm sorry - but transmission fluid doesen't go "dark" in
90 days. If it was "dark" it was "dark" when the dealer got the van, and they probably only warrantied it for 60 days because they guessed that it was about ready to fall apart. Now they are going to pretend there's nothing wrong with it so as to stall you as long as possible so by the time you figure out they screwed you, it's a year later and you can't take it back and threaten to tear off their head and shit down their throat.
I think the dealer is going to keep nickel-and-diming you until you wise up. "start with the solenoid pack" indeed!!! How about "FINISH" with it? Go back to that dealer and ask them to tell you how much it's going to cost to permenently fix the problem to the point they are willing to put a year warranty on their fix. I'd bet money they will hem and haw and give you bullshit excuse after excuse, and if you keep pressing them they will finally tell you they won't warranty anything other than a complete teardown and rebuild. Why? Because they already know that this is really what that transmission needs, they just aren't going to tell you that until they've bled you.
If your in PoDunk Egypt, and the dealer your dealing with is the only game in town, then sharpen your claws, go back and rip them a new asshole and get them to discount at least $500 off the cost of a transmission rebuild since they obviously knew the van's transmission had problems when they sold it to you. Since they already are going to mark it up $500 more than anyone else does who rebuilds transmissions, it's not like they are going to be out anything. But, if you are in any reasonably sized city, then kiss the dealer goodbye and find yourself a good independent transmission shop.
Ted