Torsion bars and exhaust heat.

Are these XHD springs the ones with the extra leaf on the passenger side? If so, it will tend to put more load on the left front T-bar and lead it to suffer from creep more than the right T-bar.

Plus the driver's weight being on just one side will do the same. At least in this neck of the woods, it is not uncommon to see cars of all sorts with the left front sagging.

Also, check your T-bar anchor on the left side (where the T-bar locks in the crossmember under your seat) and make sure that is not failing. Rust can get to these anchors....