I have a 92 c1500 with the 4L60E auto bolted to the 350, the problem i am having is when in OD going around 65-70 mph the whole truck will shutter then stop and then do it again, it only does it around these speeds and only in OD, when in D it doesnt do it but seems the RPM are alot higher (almost 1k) then in OD, I checked trans fluid and color, consistancy, and level all check out good, not sure what the next step would be, thought of trying seafoam trans tune and then doing a filter and fluid change but have seen to many mixed reviews on it, i have also checked the u joints they are tight all mounting hardware in the rearend is tight, trans mounts are tight and in good shape, not sure where or what to check next