Can't put my car into second gear unless I'm below 5mph

Go back to your mechanic and tell him to readjust the gearshift cable and then tell him to look at second gear itself and see if the high-speed synchronizer is working correctly for second gear. And, for good measure, have him check to see if there might be bent shift rails.

It is true that after major transmission work things do have to work in so that everything will seat correctly, but, you should be speeding up after your shift from first to second and not slowing down. The fact that you have to slow down indicates that there is something going on that the mechanic may not have found when he did the work and now is trying to fudge a way out.

With that said, this is something he can’t just fudge his way out of. Your transmission isn’t working correctly, even though you had it rebuilt. It should be working correctly through each gear and not acting up. It’s quite obvious that something was missed somewhere so it is up to the mechanic to put it right — and to service it under a warranty so you don’t have to pay twice.

Just having your transmission rebuilt is expensive and time-consuming enough. You shouldn’t have to spend the time or the money to have it done all over again for something that the mechanic, like as not missed.

Or, to ask the question, when the transmission was serviced was something bent that should not and would not have been bent if the work had been done carefully in the first place. This not withstanding, your transmission should be working correctly now and the mechanic has a responsibility to make it right.

Please let me know what you find out.