I have a 99 xk8 and the parts list for the drivers side upper wishbone is jlm 21378 . on my car, it has a upper wishbone part number mja 1424 ad. can someone tell me if this part is compatible with my car ?

 

I guess the number you mention is part of the forging, it may be a superceded part number or maybe just a reference number of some sort. The part number at jaguar would have superceded to the latest listing. There seems to be a pretty long list of changed numbers.
(Not important in this case but L/H or R/H would be a better descriptor for a car made in both LHD and RHD.)

 

JLM21378 has been superceded by MJD1421AA & 1420AA depending on which side.
MJA1424AD is listed as "upper wishbone" but no reference to model / year / side.

Up to you if you chose to purchase from there, but the S N G Barratt catalogue - online or as a printed booklet - is an invaluable source of information. This is from the UK website so obviously prices are not relevant :

SNG Barratt - The Ultimate Jaguar Parts Specialist