Hi there. Assuming your car is 1.6cc stock single carb 2T engine. If there is vibration at certain speeds, could be bad tuning on the carburetors or engine mounts not good anymore.
But your situation reminds me of a same problem i had many years ago. At speeds after 40 the car would vibrate and there are rattling sounds form the undercarriage. At the mechanic we found that the universal joint (for the drive shaft/ prop shaft) is damaged and jammed. You need to jack the rear of the car, put it in gear and hear/ see the shaft is not noisy and wobbling. If it is, then you need to get it replaced. When i got it replaced i think 5 years ago, it cost RM170 with labour. Do check this first.