If this bang occurs and the whole truck shudders it's a good chance the T-case drive chain may be loose or one of the spider gears are damaged within the case. Either way get it into a service provider as soon as possible, I've seen several bad T-case failures all of which can cause alot of risidual problems if not taken care of soon. When you say you had the motor changed in the T-case I think you are talking about the Encoder, it is not in the T-case, it is located on the outside of the T-Case. The encoder motor manages the current flow channels which allow the transfer case shift control module to determine what position the transfer case is in. Check under the hood to see if the TREC fuse is blown, this will cause the encoder to not function. Have them check the planetary differential pinion gears, they may have not been timed properly after the encoder was installed, this will directly influence the output shaft bearing and drive shaft and could easily be the shuddering you are feeling when going forward or backwards. One good point to remember is that when you engage the T-case in 4low, in order to properly dissengage the T-case back to normal 4High, you need to put the rig in reverse for about 15-20 feet and then put in 4high and continue driving as normal. Hope this helps.
Cheers.
