Engine/Transmission (1998.5 - 2002) Nv4500....Popping out of third

[Cummins724]

I just installed my SB DD3250 clutch & I had a problem with my NV4500 transmission that I just had compltely rebuilt 4000 miles ago. 3-4th synchro was grinding a little... . so I had the shop replace the 3-4 synchro. Now it is shifting perfect, but it is popping out of third gear when cruising through residential areas at 25-30mph. What could be causing this?

 

[Cummins724]

I just changed the transmission mount... . it doesn't pop out of gear now, but the shifter shakes alot in third gear.