NO! please no! If you have different size rims, or even tires your VCU is trashed! Go to the previous owner that put bigger rims/tires on the back and thank him for ruining the transmission while repeatedly kicking him in the nuts.
18 inch rims are stock on TT cars. I'm not even sure 17" will fit on TT cars because of the brakes. When buying rims you have to make sure they clear the front brakes. Those rims should fit fine in 18". Some of the other members can tell you for sure.
Back to the VCU part,
Stealth 316 - Mitsubishi 3000GT AWD System
If somebody put 19" rims on the back with 17-18 on the front then the VCU is ruined, you don't even want tires to have uneven wear. Basically you have a FWD car now, the rear part of the drivetrain is useless.