My truck had a 3" suspension lift on it when the d-line was made, not sure if that accounts for the extra 3" or not.

The upper bracket wont be in the way of the new d-line, it would be nearly impossible to get the drive shaft to hit the upper bracket. You would have to violently compress the rear suspension from jumping it before the d-line would hit the bracket.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed