Any relics of a Quadratrac? Hole in the glove box? Holes in the floor for the low range shifter? By '78 I'd think a D20 in a TH400 J-20 was very unlikely, though possible. If the truck came witha Quadratrac and was converted, the shifter likely came with the transfer case. I've only seen CJs with the factory inline shifter - Commandos and FSJs always got the U-shifter, AFAIK.
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, KOs, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D