Since you don't know how long ago it might have been changed,it'd be a good idea

 

I think its probs withyour trans. You said its fine in drive and shakes in neutral? I would agree with BV if you said it vibrated in drive since there is a load on the engine while in drive. Have you been pushing the car since you used the seafoam? How many miles on it?

 

Does it happen in reverse? Have you taking back to whom ever did you flush job yet?

 

Okay UPDATE.

Just had the brakes replaced, noticed a slight vibration while going down the road. Before it was only while it was in neutral or park but now it happens while crusing. I tried pulling the wheel in and out and it doesn't have any play in it.. but I'm still going to pull the bearings out this weekend just to check. Also going to have my rims retorqued as I just had custom rims put on before the brake job and thinking the lugs might've come loose.

Also, taking the car in either tomorrow or Saturday after work to have codes tested.. I'll let you guys know then about the neutral/park vibration.. but if you can tell me about driving vibration between now and either tomorrow or Saturday I'd appreciate it.

Thanks guys.

 

Only other thing I can think of is a bad motor mount/tranny mount