I have a 73 Blazer with minor mods 35's and 2" body lift. A bit of cowprint about the truck ala gateway hence the name. Teenage daughter drives it. Problem is bad thunking increasing in frequency with speed, but close to one thunk per second. Doesn't happen in reverse in any t-case setting. Changes character with different t-case gears. Any ideas prior to towing to tranny shop???