Suspension

Not a frame problem. At this age, the coil springs can be weak. Find two matching points on each side and measure from those points to the ground, then compare them. It is okay to have a half inch difference but by the time you get to an inch difference, you know the springs are weak. Replacement is a common procedure, and an alignment is needed after that.

Half an inch is not significant. If you check ten other vehicles, you'll find the same thing. If this really bothers you, there are spacers you can buy and install yourself. The most common ones are placed between two adjacent coils and screwed in a quarter turn to hold them in place. They are meant only for minor corrections such as what you need.

There are published ride height specifications. It will list an acceptable range, not a single value. If your measurement falls near the low end of that range, it is acceptable to use those spacers. If the measurement is anything less than the specified range, the springs are supposed to be replaced.