As far as I know the ilvls will be generated in classic the same way as in lod while gambling. In lod 1.11b ilvl is between clvl - 5 and clvl + 4, with a chance of 1/10 for each lvl. The prefix +2 is alvl 90 which only spwns on amulets with ilvl 90-99. So you need to be clvl 86 for a 10% chance to get an amulet of ilvl 90. For lvl 87 you have a 20% chance to gamble an amulet of lvl 90-91 and so on.
Seralph is right, with the exception of forgetting the +2 ilvl given to you. It is -5 to +4 with an automatic +2, effectively making it -3 to +6. You can gamble one at level 84, but the chances increase with every level you get until 93.