The lift probably causes it. Lot of people dont have problems with a 2.5" lift, I did. Inspecting your driveshaft is not going to fix it, thats a way for him to collect more cash. 3 ways to get rid of the vibration.
1. cheapest but contradicts the point of a lift is dropping the t-case skid.
2. Medium price option is a motor mount lift. Can be done without a body lift by moving the fan shroud up 1"
3. The common "right" way to fix it along with the most expensive, a SYE (Slip Yoke Eliminator) and Double Cardan (cv type) driveshaft. Runs around 500 plus you need adjustable upper rear control arms. Total if you do the work yourself runs from 700-1000 with quality parts.
On the 2.5 lift, I would do a Motor Mount Lift and move the shroud up 1". On the 3.5 lift, you might could get by with a MML, but I would suggest a SYE and CV shaft.