I had the same experience as brokenhead with my Suburban. I used generic glaspacs close to the firewall. The sound was obnoxious at highway speeds. I added Walker resonators aft of the axle and now have the sound I was looking for. It has the rap going through the gears then mellows out on the highway.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.