I have developed a mysterious bump somewhere in my suspension or driveline.  The knock only happens when starting in first gear from a full stop.  It sounds like someone is lightly but rapidly tapping on the underside of the floor pan.

Jeep info - 97 XJ with 6 cyl, AX-15, corp 8.25, and a 3-1/2" RE kit.

The knock developed before the lift was installed, and I had hoped the kit would eliminate the knock; I previously thought it might be related to the old bushings (I haven't touched the upper arms yet, so it still might be this.)  I have climbed underneath and shaken every part I can get my hands on, but everything seems solid.  There's clearance around all U-joints.

Anyone have any ideas what's going on?  

Check motor mounts and Tcase mount as counter torque against them is highest at startout.  

Thanks.  I'll give them a look.  

muffler or exhaust pipe maybe also  

I am pretty comfortable with it being a mount. He has a lift and bigger tires and likely stock tall gear and this places a lot of strain on engine mounts as they are the only thing controlling counter torque. Counter torque is highest at start out and it over comes the weight of gravity holding engine down on drivers side and it "bumps".  If I felt it I could tell you for sure right away. I have had it happen to me on my Jeep many years ago.