Ok man, I'm not the main guy on anything...except stupidity...I just know what I've ran into over the past 3 years owning 3 and working on a few.
Higher speed vibrations after a lift typically point to a change in pinion angle causing the drive shaft to be at mismatched angles and not cancel itself out as normal, with opposing equal angles.
I'd first pull the front shaft, then drive it. Then do the same in 4 wheel drive with the rear out if you think it's the driveline.
Next, see if anything is loose or got changed. Really go through it.
Sometimes, I wonder ....and may be wrong....can a u joint that has ran at a certain angle for ages, then when slightly changed it may act up or be stressed where it hasn't been before..or was the shaft split at the slip joint and reassembled out of phase...this is a real possibility with some type drive shafts.
Also, was a wheel weight knocked off.
Make sure your transmission mount is solid.
Basically..it can be a number of things, but I'm not trying to say it's unsolvable...guys work it out constantly. It can be elusive, but 2.5 inches is typically not problematic.
There are certainly way more things than I'm saying...I'm just using what comes to mind...we have some great guys here... like
@jjvw ,
@JMT ,
@Chris and many others, hopefully they will chime in and walk you through the basics.
I say the basics respectfully, because I've been on here 3 years, and I'm still needing to be drilled on the basics. I made a good gain with one tonight because of going back to the basics.