Severe engine shake when gas pedal is released Options

[fribox]

Hey, I have a 1996 Celica GT which I've only really posted in the "my project" section, but I've got an incredibly weird problem that I can't figure out.
Its a little hard to describe, but:
When I let off the gas my entire engine lurches forward violently, and lurches backwards violently when I press on the gas again. I had originally thought this was a problem with my engine mounts, so I replaced all of them and it in-part solved the problem (made it much less awful), but its still very annoying. When I had it at my local mechanic he said it was something loose inside the transmission (a chain, I believe he said, but cars don't really have chains in the transmission anymore) and claimed that I'd need the transmission rebuilt for $1500 (which I'd rather just buy a used one), but he also said that it wont break on me. I'm not sure if hes just feeding me garbage to get my money, but I'd really like to fix my car. Since I can imagine this is awful for my engine I've just been avoiding letting off the pedal to engine brake, and have been putting extra wear on my clutch putting it back into gear any time I do let off the gas to stop the engine from shaking.

Any ideas?

This post has been edited by fribox: Jan 11, 2014 - 4:13 PM