The problem with Polyurethane bushing inserts on the GMT400 vehicles is that you don't get the inner sleeves for the upper bushings. Energy Suspension or ProThane, both companies omit the upper sleeves.
The lower bushing inserts come with new sleeves. But not the uppers.
I made new sleeves for my K1500. The originals were corroded to death. Fabricating them was no damn fun at all because I don't own a lathe, and the local steel supplier didn't stock the tubing size I needed. I had to "mail order" tubing, and then cut the OD down the hard way.
Photo 1. Poly insert, new vs. old inner sleeves. Both "old" sleeves came out looking pregnant. I tortured the rust off of one of them on a wire wheel, so now it looks "skinny".
You must be registered for see images attach
I'll never install rubber bushings if I can get Polyurethane. They're so much easier for D-I-Y installation, and you don't have to put the suspension at normal ride-height before torquing the bolts. Having said that...I did install "new" complete upper control arms (that come with bushings and BJs) on the K2500; and this summer I'll do the lower bushings and BJs. I just wasn't willing to make new upper sleeves for the K2500.
Eventually, I gave my excess tubing to my Machinist friend, he cut the OD on his lathe for me. (and kept one set of sleeves for his trouble.)