Misterk3;
You NEED to find load range E tires to put on that Excursion. Originally these guys shipped with load range D and then were hit with I believe a class action suit where they were changed to E's. Going over the scales your empty Excursion will weigh more than an F250 CC. If your only putting in 40psi I wouldn't be sure you even have range D tires on there and the sidewall flex will be scary.
The spring rate on the Excursion is not the best, but I will tell you I towed my 8000lb travel trailer last summer a few times without any suspension modifications and it did "decent". I have since upgraded the hitch to an Equalizer 1,000lb WD hitch, Hellwig rear sway bar, airbags, KYB Monomax shocks and new load range E tires.
World of difference.