the kits can definatly be done in the garage, theres nothing to weld. Just replacing things like springs, shocks, and depending on the kit you buy maybe control arms.
They say the SYE is required but instead of doing that to save some money you could install a transfercase lowering kit and a motor mount lift with the 3.5. You may be able to get away with it, not promising it will get rid of vibrations. Althought the sye and cv shaft are the right way to do it if its in your budget.
With the 4.5 I would def. get the sye and cv kit.
When I do my 3.5 im just gonna do the TC drop and maybe the motor mount lift while i save money for the SYE and CV