Hi, I have a 99' 250 Superduty with an 8" lift. I am considering lifting the front 2" more. However, I was told that the drivetrain angle is already too steep and that this could cause a problem. My question is: what type of modifying (drivetrain, brakelines, etc...) should be done at this point?

 

At 8 inches you are at the threshold of all of your angles. The rear drivshaft center support bearing will have to be lowered (again) and your parking brake cables, and VSS harness are starting to reach the limits.
If you still have the stock front brake lines and still have your ESOF hoses connected, you should be considereing some longer ones.
Moving towards the center, the front driveline is now going to be too short and will run the risk of popping out of the slip joint. An extension on the flange might cure this, but with any kind of suspension extension, it still runs the risk of failure. That being said, the driveshaft may need some modification, and could probably use a longer one.
If you still run a sway bar, then some new extensions may be required.
Your drag link and trac bar have reached their limits so some attention wil have to be paid to these. The drag link has become rather short for this amount of lift and the angle is getting rather steep, so the consideration of true crossover migt be in the cards. This is going to require a new steering knuckle to get the angles under control if this is going to be done correctly.
Now since the front driveline is at such an angle you will have to correct the pinion angle, but these too is getting to the point where an expensive "c" rotate is going to become necessary.
Now, this sounds like a ton of stuff, and yeah, it is, but the 10 inch mark for a leaf sprung truck is right about where things start to get real expensive.