HELP!! will not go into reverse in 4wd

I just had a bunch of work done on my 95 XJ (stock auto and TC) including the installation of a 3" lift, a tune up, new hoses & thermo, rear brakes, some gaskets and seals, complete fluid changes, and some more stuff... everything seemed to be ok until the mechanic was test driving it and realized that it would not back up in 4wd hi/lo... backs up fine in 2wd and goes forward in 2wd and 4wd but just goes KLUNK! when put in reverse while in 4wd... this did not make sense to him (or to me) and our best guess is something in the TC but even that seems odd if it still moves forward in 4wd... anyone seen this before? I've heard that there are some linkage issues with the TC in this era and the trans shift lever is pushed all the way to the end of the slot in the console but it was like that before any work was done and everything worked like it was suppose to... the only work on the TC was to patch a crack in the case with some cold weld...

A crack in the case?! It sounds like the chain in the transfer case is bad thus binding up on itself. Where was this crack at on the case?

Thanks for the info... several people have mentioned this and suggested that the wear on the gears themselves would make the problem worse... makes sense! now I have to decide whether to buy a rebuild kit (Crown or JB Conversions) with a chain; find a used TC and just stick it in; or upgrade to something like a JB Conversions 231 with the SYE already installed... if I upgrade then I'll have to find a better rear axle (its a stock 8.25) and regear the front and it would probably be smart to go with a locker while I'm at it.... $$$$$

or you can buy a used one at a junkyard