not so good vibrations?

[yellowcel]

I've been driving my '90 gt(automatic sadly) since late '06, and even before I got the car it had heavy engine vibration that is related to rpm. The engine is operating properly(no knocks or anything), and it shouldn't be the trans since the vibrations are just as violent in neutral as they are at the same rpm while in drive. I've been trying to sell the car, but it looks as if it will stay in my possession and become my long term project, so I would like to diagnose and repair any major problems(I've finally got most of them done, this is one of the last that interferes with comfort and drivability). A friend suggested that it could be a broken balance shaft, but I cant find anything in my manuals about balance shafts. I thought that it could be a balance problem with the crank being damaged somehow, or the pistons being far off balance(not sure that piston weights would have any major effect outside hardcore racing, but its worth a shot). I will likely do a five speed swap and rebuild next spring or summer, so if anyone has an idea I would be glad to hear it. It may be worth noting that I had test driven several celicas from this year, all of which had a similar problem.


Also wondering if anyone knows where i could find a gt4 front bumper, the one with a large intercooler opening. I've been searching forever, but the only ones I find are rediculously expensive.