hi i have a 93 ranger sas'ed. its got a 8 1/2'' rubicon expess lift and when i drive about 35-40mph in 3rd gear it shakes really bad until i speed up or slow down. 35-40 is the sweet spot to get it to shake. does any body have any idea as to what it can be. oh ya and its whole drive train is out of a 98 ranger.

 

Could be anything from improperly balanced drive shafts, wheels, bent axels, wheels out of balance...