The only difference in mid 99 that I am aware of is wheelbearings and brake rotors. A lot of people have told me late 99 XJs (NOT grand cherokees) got the coil rail and new head but every time I ask for VIN verification it turns out it's a late 99 production date and a 00 VIN, thus 00 emissions system, which is unsurprising and NOT a mid 99 split.
The VIN year is the one that matters when talking about model years, the production date is sometimes used to narrow down early vs late but the split isn't always in the same place and you have ot use your ears and eyes, they aren't the same, production date year matching VIN year is more rare than you'd think.
Is it a WJ Grand Cherokee Laredo or an XJ Cherokee Laredo? If it is a WJ, it could be a coilpack issue I suppose, they all got the new ignition system in 99. If it's an XJ it should have a distributor and I'll be shocked if the coil would cause a single cylinder misfire as a result.