Age and use cause the stock rubber bushings to degrade and cause excessive wear potentially breaking the housing and or driveshaft.
These upgraded polyurethane bushings hold the differential tighter than the stock bushings but not rigid where things have a propensity to break. If you notice clunking or excessive rocking in the diff consider picking up a set.
EACH KIT IS COMPLETE FOR 1 VEHICLE. 
Colorado/Canyon has 2 bushings on either side of the front axle.
Hummer H3 has 2 bushings (one on each side) up front, and one larger one on the crossmember.
Check out this install video before purchase so you can see how these go in. https://youtu.be/ASVf_Yscsqo
Check out the rear h3 crossmember bushing install in this video
https://youtu.be/onWV8DDCUd8