something getting loose in the drivetrain

[Chamberpains]

If it's 1 of the rears, check the oil for metal shavings or little sparkly particles all thru it. Jack up the axle and spin the wheels, you'll hear it clunking and grinding. If it's to the point of "catching" and clunking then it's already way past the point of saving. You will have metal all thru the oil and out to the wheel bearings. You will need to replace the rear end and wheel bearings and races.
Most rear ends or differentials in big trucks are remanufactered and prebuilt. You just pull the axles out and unbolt the rear section from the axle housing, then pry it out. The front rears or power divider have to be slid off the thru shaft on it's way out and slid back on thru shaft on it's way back in, but is simple enough to do. Just time consuming and messy to clean all the shavings out of the axle housing. If you've done a wheel seal before and look at the rear and say "ya, I can wrestle something that big" than don't be afraid and do it yourself. But have a friend handy to help with the wrestling and lining things up.

This is a how most power dividers or front rears look.


And this is what most rear rears look like.