I'm under the impression that being too specific is a bad idea in dependent claims. But is there a case when I really have to be, and therefore cannot use "further comprising" but rather I must use "wherein"?
Dependent claims are the place to get narrower and are often chained to get more and more specific as they descend. The use of "further comprising" vs "wherein" is unrelated to the extent of narrowness you are adding. The parent claims requires an A and a B and a C.
"Further comprising an X "says you are adding a new element, X to the already required A, B, & C. "Wherein C is blue" gives more detail to an element already required.
Claims are often arranged in a hierarchy of increasing specificity and included structure. To infringe your patent an apparatus only has to fall under one of your claims.
If claim one says "A portion of a vehicle comprising an xyz transmission coupled to an abc drive shaft." Claim two might say. "The portion of a vehicle of claim 1 further comprising a two-wheel drive system." And claim three might say "The portion of a vehicle of claim 1 further comprising a 4-wheel drive."
In both claim 2 and 3 something is added to the elements of claim 1. This only works because claim 1 was patentable on its own. If some complete drive train is needed to get a patentable claim then claim 1 can say "a transmission coupled to a drive train". Claim 2 can be "the thing of claim 1 wherein the drive train is for 2-wheeled drive" and claim 3 can be "the thing of claim 1 wherein the drive train is for 2-wheeled drive."