I have a 91 extra cab long bed Cummins. when braking around 35 mph I get the weird hoping and it doesn't feel like brakes, it feels more like the chassis flexing. is this the case? is anyone boxing their frames? or it something else?
does not to it under acceleration.

 

What is hopping?
If it is the rear end ,it might be the ABS system pulsing or grabbing.

 

Don't think it's chassis flex I think it's your ABS brakes acting up . There is no need to Box the frame in .Just a Waste of Money and Time .

 

thanks guys, I didn't even think about the abs system. How would I go about checking that? is there something I can unplug?

 

Drop the glove box down and there is a Black box just unplug it . See if that help's .