I have a 99 TJ with 4" lift and every time I take off up a hill I get a loud popping noise right under my feet. It feels like something is jumping up and down under me. It also dose it when I take off real fast too. But once I get going it stops It only last for a few seconds anybody know what this could be

You suck at providing information :)

What 4 inch lift? Control arms changed? Stick or auto? TC drop, or SYE? Any issues in reverse? When did this start? Right after lift installed?

I have had the 4" lift kit since I have owned the jeep 2 years. It started when I was wheeling in 4WD going up a steep incline just as a small popping noise and has gotten worse here lately I have a 4 cyl engine no sye I had not done any upgrades when it started now it does it in 2WD. It only does it when I am taking off. It is real bad when I am starting off on a steep incline. But it also does it when I take off on flat ground but it is not as bad. The tranny is an auto. I have also notice that the Tcase shifter bounces up and down when it does it. I dont have any problems in reverse

Check your trans/t-case rubber mount, it may be wore out. When you give the Jeep gas from a stop the engine torques everything to the right, thus kicking up the left side of the drivetrain, which would explain the t-case levers moving around. Hope this helps.

mine also pops sometimes
but mine is the trackbar
sometimes it comes loose and the pop is made by too much play

I have a 99 TJ with 4" lift and every time I take off up a hill I get a loud popping noise right under my feet. It feels like something is jumping up and down under me. It also dose it when I take off real fast too. But once I get going it stops It only last for a few seconds anybody know what this could be try this, while accelerating hard enough to make the noise, GENTLY pull back on t-case shifter, just enough to slightly preload the linkage. There's a bushing that wears out and creates a noise like someone tapping the trans tunnel with a hammer intermittantly. If the bushing or shaft is worn, it should stop the noise when you preload the lever.
here's the p/n for the repair kit
**1 05014148AA Repair Kit, Transfer Case Shifter(TJ/XJ)**
HTH!