Speedo bounces after 110 km and when bouncing will not let my cruise control work properly . Problem is on a '91 Chevrolet Silverado 1500. It is a 350 with automatic and four wheel drive. Only has 107000 original kms. Wondering if there is a way to tell if it is the speedometer sending unit or the gauge cluster. Local chev "techs" have not been too helpful.

SOURCE: 2004 Chev 1500 Suspension bounce/jiggle

There is a Service Bullitem out for that year (2wd) because the front frame cross member needs shims. I had it done to my crew cab 1500 a couple years ago, but the noise is back now :/
I'll look for the SB# a bit later. Good luck.... should still be covered, I think.
Don G.

SOURCE: fuel gauge k1500 98 Z71 fuel gauge vibraiting or bouncing

gauge failing (dash)

SOURCE: 99 Malibu all of a sudden my speedometer started bouncing wildly

I'm not 100% sure but your problem may be a speed sensor.  This is just a sensor that monitors how fast the tires are moving.  These are usually easy to change.  They require usually one or two bolts to be removed and an electrical plug.  But as to the location of yours, I'm not 100% sure, but I believe it mounts somewhere on the transmission (transaxle.)
Any questions, just ask!!! Please rate!!!  Thanks

SOURCE: 99 chev k1500 5.3 liter at idle with all acc

NO, IF I REMEMBER RIGHT THATS EXACTLY THE WAY ITS SUPPOSE TO BE, ITS MENT TO HELP YOUR FUEL ECONOMY. IF YOU DONT BELIEVE CALL DEALER ITS FUNNY LISTENING TO GO THROUGHT THE HOLE SPEEL. THISINFO COMES FROME A DELCO SEMINAR

SOURCE: i have a 95' 1500 chevy pickup ... Cluster issue

sounds like you may have a problem with your speed o gear in trans.