I'm sorry to have left in the dark
all the contributors to this thread for almost two days. So this is the latest news...
I tried to go to the closest Sheriff Office on Wednesday night, but since they close at 17:00, it was already too late. Anyway, as mentioned in my previous post, I was able to link the frame number to the others on the body, so I do have a 2002. The only swap, so far, is the engine (but I haven't checked the trans yet).
Yesterday afternoon, I send an email to the dealer. I explained the problem I had (the 4,000 rpm thing), the discovery and verification process of the 99 engine and the fact I was pretty upset. I also explained I already contacted the DMV, the State department of Consumers Affairs and the Better Business Bureau. I told him that I was "sure" it was a mistake and I was looking to give him a chance to explain himself about the problem, before I contact a lawyer and go "legal".
Four hours later, I received an "out of the office" message, stating he was out until August, 1st. I know these things take only few minutes to come back, so I'm guessing he was trying to buy himself time. All right, let's say he has a slow server... But his message was also stating that I could contact the guy in charge, the parts man, if needed.
So this morning, I called the "part guy". I explained him in details the situation and told him the dealer has until Monday to contact me back and explain himself. Else I will contact my lawyer on Monday and we will go legal. He told me his boss was out of town already, but that he will call me back quickly.
About 60/90 minutes later, the dealer called me back from his vacations location... And basically (and to summarize):
He is the one who did the swap. He bought the engine from a salvage yard. It's a 99 engine with 54,000 miles on it. He can provide (or have me provided with) an official document showing the VIN number and the mileage and he will pull out the Carfax of the car, if I want to.
He states that the original engine has a big leak of oil, and apparently the previous owner didn't knew he had it (he mentioned something about the oil pan). He had to change the engine because it was already half toasted (or something similar... Dead, unusable).
He totally forgot to mention about the swap because he had, at this point of time, so many things going on and it was a crazy time. He had a lot of Jeeps going in and out and couldn't remember what has been done on each of them (I think this is a little bit of BS, but let's say I believe him).
I asked him to provide me with an official document explaining why the engine was changed and who did it (A specialized Jeep dealer) and he didn't say no.
That we talked about the 4,000 rpm problem and he said that it was probably not related to the engine he swapped but probably a problem in the fuel lines or the O2 sensors (original parts of the Jeep). Anyway, he offered to fix whatever the problem was at no charge, because he want me to be happy with the car. But also he owes me one (it's true but a too long story, not related to the actual issue, to explain).
And he asked me if I could wait until he comes back from vacations, because everything goes wrong when he is not here and he'd like to personally supervise the whole thing.
(to be continued)