I've noticed since i got my car that when im accelerating (not hard just more of a steady run, gears change around 2000 rpm) that when i get to 60 n up to about 64ish the whole car kinda.... shudders? its def coming from somewhere in the back and i was just wandering if someone else has or has heard of this same problem.

She's an 87 iroc with 110xxx miles and i was told that the previous owner had changed the gears ( to 3.73) i believe. Ohh and while im asking questions at wat speed does fourth gear happen? Umm at 60 mph im doing 2000rpm amd i've had her up to 115-120 before and was still in third gear. Any ideas?