Dave, with 525 lb. in the vehicle, I think it should have maintained 60 mph without a forced downshift to third. If not, then definitely check the boost.

Pick up a boost gauge on e-bay for $20.00. It comes with a length of plastic tubing that connects to the back of the gauge.

You will need a two short pieces of vacuum hose that fit the end of the tubing. You will also need a plastic "T" that fits 4mm vacuum hose.

Go under the hood and find the line from the overboost protection valve (on the firewall) to the ALDA. Interrupt this line and insert the plastic "T" using one of the short pieces of vacuum hose. Connect the plastic tubing from the gauge to the "T" using the other short piece of vacuum hose.

Route the plastic tube under the hood and around through the driver's window so that you can watch the gauge while you drive it.

Take it for a ride and watch the gauge when the engine is fully loaded and above 2500 rpm in high gear. When set properly, they will be at about 11. However, on older engines, they never reach this value anymore. My SD runs at 8. The wastegate spring can be readusted to get the 11 back, but, I have not done it as yet.