I performed the same test on my '92 (3.73 axle ratio-31" tires). At ~30 mph
manual 1st gear -- 3100 RPM
manual 2nd gear -- 2100 RPM
D -- 1400 RPM
I looked up this
spec sheet on the A4LD. Taking those gear ratios, I can calculate what the engine RPM's should have been based on 3100 RPM in 1st gear.
2nd gear should have been 3100*1.474/2.474 -- 1800 RPM
3rd gear should have been 3100*1/2.474 -- 1300 RPM.
Someone check my logic on that. Does that seem right?
Anyway, it looks like the transmission is at least shifting, though I'm inclined to say that it looks like something in your transmission is slipping.
Have you checked the transmission fluid level and condition?
The next test might be to perform a similar engine RPM vs. gear test, but at a higher speed so we can check if the O/D is engaging. Try manual 2, D, then OD at 40 mph.
Another test might be a stall test on the torque converter. put the transmission in gear, left foot on the brake so the truck doesn't move, then romp on the accelerator and note what RPM the engine settles on. Don't hold the accelerator down for more than a few seconds and let the engine idle in park for several minutes afterwards to cool the fluid down. I don't have a manual in front of me, but it seems the stall speed is supposed to be around 2500 RPM. If the engine gets much above 3000 and stills seems inclined to rise, then abort the test.
I hope it helps, and good luck.