something knocking under me

ok so about a week ago i noticed a distinct knocking vibing through the floor when i accelerate from a stop and when i hit small and sharp bumps. i figured whatever it was it would work itself out, jeeps like to make noises. now i'm worried. its not the ds i'm certain, at least i think i am. the only other conclusion i have come up with is by waiting to locate a 44 before gearing down, my 33's have managed to chew up the stock gears in the 35 and they are skipping when i initially hit the gas before they catch. ???????? does that even make sense. please help me out here, any ideas or thoughts.
TIA
Rockhound

It would help us a lot if you tell us what year, what engine and tranny, and how many miles on the odometer. However -- the transmission mount in XJs tends to collapse after a few years, and that lets the exhaust system bang around a lot. Check your transmission mont and your motor mounts.

Eagle ( and anyone else )
my bad.
1991 XJ laredo 4dr, 4.0l ho auto, 6.5" with 33x12.50s, @ 170k

sounds liek a motor mount, tranny mount, or broken exhaust hanger (the one near the crossmember)

Can you tell if its from the front or rear? Control arm bushings?

Rev

Rev, no i can't tell yet whether it is front or rear. i'm pretty sure all of you guys are right about the tranny mount though cause when it happens, it feels like its right under me. i'll check all my mounts out today.
if in fact it is, where would you recommend getting the replacement? i know rustys has them and they are right by me.
how difficult is it to change these out?
thanks guys
Rockhound