Whole car shudders at 20mph (searched already)

[downbyfive]

Hey all, tried searching on this, but to no avail...

When I accelerate, the whole car shudders severely from about 18-22mph. This happens in 1st, 2nd, and even 3rd gear, all through the same speed range. If I decelerate to 20mph, then try to speed up again, the car shudders the same as if I accelerated from a stop. So basically, any time I accelerate from around 18-22mph, my car shudders like crazy, then is smooth again once I'm out of that speed range. Any ideas?

Car is 1991 240SX SE w/HICAS, 110k miles