Sounds like you may have a few factors against you.
1. As pointed out already, the Timbrens should not be hitting the axle when unloaded. Supposed to be at least a 1" gap between axle and bottom of Timbrens. If not, call them for assistance--lifetime warranty.
2. Yes your springs could be shot/weak. But w/ a 9200 lb GVW, truck shouldn't sag more than a couple inches as mentioned too.
3. Pin weight is waaaaaaaay more than advertised. Are we to assume that 5er is empty?
4. Whole heck of a lot of weight in front of trailer axles, above truck bed, in front compartment, basement?
What kind of 5er and it's specs?