Weird Vibrations 98 Z 71 Chevy Silverado

[MyZ70FUN]

I have a 1998 Z 71 Chevrolet ext cab 1/2 ton with auto matic trans with 3 inch body lift

The problem I have been having for the last couple of years is when I am in 4th gear with my foot on the throttle and my RPM's are between 1500 and 2000 my whole truck vibrates or shakes, it shakes so bad it shakes my out side mirrors.

The feeling is similiar to running over metal grating in the road at the same time this is happening my RPM"s jump up about 150 to 200 rpms.

 

The things I have replaced

New rims and tires

New heavy duty off road shocks on all four corners.

all my u joints except the two that are used when my 4 wheel drive is engaged

new fluid and filter in trans

new crush sleeve and royal purple in my rear differential (all bearings checked out good in rear end)

new diiff fluid in front axle case

new trans fluid in my 4x4 transfer case

 

any help would be greatly appreciated !!!!

[GM-Tech]

Sounds like a torque converter clutch. WHILE it is vibrating, keep a steady throttle with one foot and touch the brake with the other and see if it goes away.