Squeaking/slight grinding with full load

What is your question?

I have a 2000 Silverado 4x4 and whenever I put a big load in the back of my truck there's a squeaking coming from underneath and somtimes a slight grinding noise I replaced the rear u joint at the axel but it doesn't seem to have fixed it. Any ideas?

What makes this problem better or worse?

Less load is better. Heavy load is worse

How long has your 2000 Chevrolet Silverado 1500 had this problem?

A couple months