There's a cable that comes up from the passenger side of the transmission to the throttle linkage just ahead of the oil filter housing. Disconnect the ball and socket end of the cable from the throttle linkage and go for a drive. It should get to top gear fairly quickly. If so, your low gear woes stem from a misadjusted or more likely binding bowden cable.

Your power steering fluid is low. At operating temperature, the fluid level should be at the bottom of those pointy things in the yellow plastic stem, about 8mm higher than it is in the picture. It should also be yellowish hydraulic fluid rather than red like ATF but that's open for debate.

Sixto
87 300D