If it still has the 722.3 or 722.4 transmission, that is probably normal. My SDL runs ~3100 at 75MPH. Keep in mind that the mechanical transmissions don't have a lockup torque converter, so there will always some some degree of "slip".

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)