No Low in Drive - Turbo 350

[coupla48s]

I think my tranny is getting ready to take a dump.  Noticed a few days ago that it is taking off in Second from a stop.  Also noticed occasionally when sitting at a stop the truck shifts from First to Second while sitting, kinda like getting a light tap in the rear.  It will manually shift into Low and can be upshifted into each gear successfully from Low and Low-Second.  Fluid level is good, doesn't smell like burned toast.  Please tell me it is just a simple external adjustment involving a wrench and/or a screwdriver.  BTW, this is in my '48 Chevy pickup with a 305 SBC in front.  Thanks in advance for your help in this.  Gotta keep it reliable; it's my daily driver.

Kevin in Seattle