First post, didn't find much using the search function- if you guys know some please link them..

1995 4runner 3.4l swap'd, automatic 4 speed, 4.88 on 31's

Recently had a the transmission rebuilt after a hard trail ride cracked the case. (trans shop said the drive shaft became unbalanced, causing the crack). Trans shop told me i shouldn't b driving over 70 mph? I know these trucks are not race cars, but 70mph? On long trips i sometimes hit 80-85 on highways.
Wondering what speeds others drive at, and will driving over 70 really hurt the vehicle?
Has anyone else heard this?

Thanks