most tj's won't experience vibes after a 2" lift -whether with spacers or with 2" springs. it all depends upon your drivetrain combo though. the touchiest of combinations seems to be the 4.0, nv3550, np231, and d44 axle. that combined will yield the shortest rear driveshaft and allow the vibes to develop with the least amount of lift.
most people, when they experience vibes, will install the 1" transfer case drop and never look back. but with only a 2" lift, they are just entering the realm of vibration...and can usually alleviate the vibes with only a 1/4" to 1/2" of drop. i would try placing a washer or two in between the transfer case skidplate and the frame before installing the full drop. this way you'll only be lowering the skidplate as much as neccessary instead of losing the full 1" of clearance.
the 4wd dealer may have been trying to say that by installing the 2" spacers on your jeep you would only be around 1 3/4" height over your jeep's stock height (remember, your current springs have sagged) and possibly not inducing the vibes. the new 2" springs, however, would lift your jeep a true 2" over stock height and putting you in the area where you may starting feeling vibes.
good luck with it!
Mine - '03 TJ RE4.5 coils, long arms, 4.88 gears, arb/detroit, 35" BFG KM2's, 9.5ti, skids, etc.
Hers - '01 TJ RE4.5 coils, 35" BFG's, Hs9500, HP30/8.8, 4.56 gears, skids, etc.