I currently run a 2.28 mated to a 4.70 case doubler,any issues running highway speeds say 70mph and up with a doubler?I drive my rig 30 miles to work but might take a 200-250 mile trip outta town and want to take the Toyota.

 

No issues. Plenty of people do.

 

umm....you will ALWAYS have the option to shift into 2HI for regular driving. the benefit of dual cases is the ability to achieve 20 forward gears and 4 reverse gears.

 

I don't think he's trying to drive in low or double low on the highway.
I think his concern is wether or not he'll damage them or create excess wear.

 

I drove my junk pile from home here near San Fran to the Canadian Rockies and back last summer , averaged 60 - 65 mph . No issues with my dual cases , I have the same set up as you , 2.28 with a 4.7 behind it ...

 

been driving my duals on the road for 5+ years...about the last 1.5 years as my DD..no issues. I just change the fluid a little more often